I know that RPGs employ eye candy tactics to players of their preferred sex in order to entice the players into the game.
Male Wizards are sexy and muscular. Female Wizards are sexy and have big boobs.
Aside from that, is there any other practical explanation why some wizards are so hot and sexy when most of their time is spent on studying rather than going to the gym?
Glamour is the first spell they learn!
In real life, wizards all look like you would expect - skinny spindly bookish guys, chubby balding bookish guys, unkempt old dudes etc. The first thing they do when they get some magic is get glamoured up, buff and Harlequin romance ready!
If you are familiar with the internet, you will understand that the hot sexy female wizards are also actually one of the male types described above.
One potential mechanism, cribbed shamelessly from El Goonish Shive is pretty straightforward:
Magic is intelligent and has ideas of its own, and it wants to be used. In people with sufficient aptitude, it is so concentrated that it can affect their physiology.
Magic is interested in making more people with magical aptitude, so it can be used more. It therefore guides the development of people with that aptitude to make them as sexually appealing as possible in an effort to encourage their successful procreation.
Wizards of either sex, therefore, have exaggerated secondary sexual characteristics and an appealing body fat ratio without having to work for it, because magic wants them to be good breeding stock.
Disguise
We have all the stereotype that a wizard has a long white beard and a pointy hat, and maybe a pair of glasses, while a witch will be a not so nice looking woman, dressed in a black robe and with messy hair.
Now, imagine you are walking in a forest and you see a gentleman with a long white beard and pointy hat coming in your direction: you think he is a mage and evaluate your chances of successfully defending yourself should a fight arise.
Imagine now that you see a medieval version of Pam Anderson running (with all the bouncing) in your direction: will it inspire you fear or something else?
To sum up: in a world where everybody is a potential menace, disguising your true potential and skills is a valuable skill. Mages are known to be dangerous at distance, so having a chance to get close to a potential target is also a valuable tactical advantage.
Attracting new practicioners.
Does every basketball player achieve Jordan's success? Do you expect Ronaldo's luck with X brand shaving razor?
It sells, so if the Grand Maestre tells you to pick some members for the next fair, you ditch the fat enchanter. No one will join the Alchemist's club if the gal on the stand has pimples and nails-on-chalkboard voice.
Pick a sucessfull Spellweaver who has looks and brain, with a passion for the arcane. Then your next year the kids will beg to enroll. More students = more money from the crown.
If it brings more money, why not?
Practicing magic is a strenuous exercise.
Magic is a powerful force. Summoning and guiding it via gestures is a physical exercise. Learning spells in the library is not all there is to being a wizard.
Nearly every Manga uses this to explain the protagonists' hulking bodies.
Being able to control magic is used for selfish means.
Who wouldn't optimize their body in terms of physical appearance, simply if they could?
Seriously. Pick all the forms of magic in the real world and at some point they delve into it. Some are all about it.
One of the most known group of practicioners of magic on Earth is the Order Templi Orientis, made famous by Aleister Crowley. Those guys organize their knowledge in degrees, and the higher ones are all about sex magic. They consider masturbatory magic to be the lower form of sex magic, with intercourse based rituals being higher in nature. And you need to be well versed in it these techniques in order to make it to the top of their hierarchy.
In a world where magic is a thing and you get power by doing it like mammals do in the Discovery Channel, a sexy body is a tool to obtain ever more power.
Isn't that obvious?
I always marvel at wizards in games, including roleplaying and LARP. They all don't seem to think like real people. Because when you learn something new, you don't change your personality, do you? When you learn a new craft or language or anything, does it change what you want in life? Maybe a little, but not fundamentally.
IMHO some of the first things that anyone who more or less suddenly finds himself in possession of magic powers would do is - the same things he would do without, just with magic:
The first thing you do, especially in a medieval/fantasy world where hunger is a real thing and most people know how it is to go a day or two without food, is to check how you can use magic to make sure you are never hungry again.
The second thing would be to ensure you have a proper roof over your head, preferably with a heat source. Magic flame? Favorite spell!
And as soon as you are comfortable and safe, of course your mind turns to "how can I use this new thing to attract members of the opposite sex?". Healing spells, especially if they can restore your teeth, skin, etc. definitely rate high on that scale. If there are spells to shape your body, you will definitely use them. The good food and warm shelter from the first steps also helps to get and/or maintain a good body.
These things are what biologists call fitness indicators and if you can prop them up by magic, you will. Especially if you are a young adult under the influence of hormones.
Who wants to live forever? Wizards, of course.
The only issue is that they have to spend most of their time studying and practicing magic, while other normal people go to the gym and mate other people, but they know that their time will come.
After years and years of dedication they finally master some branches of magic and they can have their payback. Given that several enchanters can perform a full shapeshift it's not a big deal to modify their own appearance to meet the current beauty standard.
It's not a lazy way, they spent years on their books, and it's way more smart to work out in a library rather than in a gym. A bodybuilder will grow old, a wizard will grow powerfull then, granted intelligence is mandatory to become a wizard, they totally won't spend their time trying to sculpt a mortal, frail, aging body, but they will do their best to increase their power (and therefore their appearance, that can be modified as they wish).
There's actually a downside in this behaviour, and it could be a relevant downside especially where there's a high wizard concentration: a wizard will end up mating another wizard, since only another wizard can match his/her beauty. That's a problem because a wizard will recongnize another one and he/she definitely won't to "create a family" with another wizard, and with their power it would be easy to control their fertility. On the other hand the not-wizard people are not that appealinig, except for few really special people. This actually is the reason why, in the end, wizard are so rare: they can have as many one night stand as they wish but they luckily won't have offspring.
Ps: since a wizard can modify his appearance and he is intelligent, the "Adone" shape will be used in "commercials" and saturday night but, in order to avoid possbile backfire by gealous ex lovers and/or king's discomfort for the wizard's influence on the queen, when they do their "official" business they will use a "Merlin" shape. They are paid for their wisdom and power, so they are just maximizing their appearance again to have the maximum gain.
Aren't they supposed to be really intelligent after all?
This is similar to the glamour idea from @Willk, but different in that they don't have an illusion around them.
Everyone has a certain magical energy and potential in him. This energy is constantly leaking out. Have you ever thought that someone has this kind of aura around them? That's not simply their personal attitude, it's more like magical pheromones. The more magic, the stronger the pheromones that tell you: "This person is sexy."
Your perception is skewed into thinking they are extremely sexy, when in fact they are quite normal, but with an enormous amount of power leaking out of them and telling you that they are exceptional individuals and mating with them would be good for the bloodline, because:
magic = power = survival
Because of cognitive dissonance your brain will then proceed to rationalize why you feel that these ordinary looking individuals are so extremely attractive. And the easiest thing is to overlook everything that doesn't align with the typical characteristics or what society defines to be "sexy" for the average non-magical person. This makes you think they have sexy bodies when in fact they don't.
This way there is no awkward morning breakfast after forgetting to put your glamour back on. It's always on as long as the magical energy is not suppressed by something. But the moment it is suppressed the individual will look quite plain compared with the image humans had in mind.
This also makes wizards extremely weird for normal people and might help with the perception of them being different. While the general view of biology on what defines "sexy" is ingrained into the minds of people there are differences that are defined by society and "sexy" can also differ from individual to individual - which means that different people might desribe wizards slightly different. Simplified: one group of people talks about the upper body, while the rest talks about the lower body of the individual wizard. This leads to people not being able to properly describe the wizard and therefore leads to a aura of mystery around these individuals, which increases "traditional sexiness" yet again. They are rare and complicated individuals and nobody is completely sure what they look like, but everyone knows they are extremely sexy - whatever you think sexy means.
Mens sana in corpore sano
The manta of wizardry is mens sana in corpore sano: the younger wizards wish the attract a partner, the older ones wish to stay alive, all have a direct incentive to stay healthy.
Therefore, the first interesting spells studied by wizards, straight out of basics (aka small telekinesis, dust cleaning, ...), are basic body reinforcement spells:
Masters know it's important, and know their young students are keen to learn it, so they teach it first and use it to convey the fundamental concepts of magic.
As a result, all wizards just know how to maintain a healthy body.
Actually, an unhealthy body is a mark of insanity for wizards, and history has proven that those wizards who let their bodies degenerate soon devolve into villains.
Inherent "Healing" Ability
Magic is good for the body, and promotes health and well-being in practitioners of both sexes. Minor ailments tend to pass them by, and even major issues will heal completely given time. It could even help them maintain a healthy weight and muscle tone despite lack of exercise.
So while they mightn't be super attractive, they do tend to have clear skin, good hair and teeth, be fairly fit and have that air of confidence that comes so naturally to magic-users.
Magic is sexy!
If you're only interested in out-of-world opinions, then the definition of "sexy" comes from our culture, and will need to be tailored to our particular concept of "sexy." However, if you think in-world, if magic is something desirable, any body characteristics or behaviors associated with magic will develop an element of sexyness to them as they become desirable in-world. Whatever magicians have, the opposite sex will want.
Related: reading is sexy
Look at The Witcher world, in this world sorceresses look hot while Wizards usually look old and frail even if they are young, they can change their bodies with permanent enchantments and they do it because their clients expects them to be hot for women and old and fragile for men, also the sorceresses tradition usually favors having ugly apprentices so they work hard to master magic so they can change their appearance so they can stop being mocked by others.
Also after someone masters magic they lose their means of reproduction so they can fool around all they want without consecuences (also they are unable to catch STD) so being hotter is even better for them.
And for wizards not all of them look old, one of the books antagonist is a wizard that looks young and is quite muscular, to the point that it can go 1 on 1 with the protagonist (who is mutant that hunts monsters) without any problem.
So maybe the reason in your world both wizards and sorceresses look like super models or porn actresses is because people have that precognition that the best wizards and sorceresses should be hot because the magic to change your own body is hard to master so if one can change its own body means that are truly masters and not lowly apprentices.
Two reasons, somewhat related.
First, wizardry takes energy, some of which (the part that controls the rest) has to come from the wizard's own body. Thus, the more physically fit the wizard is, the easier it is to control magic, and the more powerful the level of magic the wizard can control. So a successful wizard pretty well HAS to be pretty muscular.
You might, for instance, think of doing a magic spell as rather like push-starting a car. (Something that those of us who've owned '60s British sports cars¹ are all too familiar with.)
Second, part of wizardry involves heightened control over one's own body. One can heal diseases, and within limits shape one's body into a desired form. So while it isn't as simple as waving one's magic wand about, wizardry augments the effects of exercise, so that the wizard gets a buff body with much less gym time than normal folks.
PS: WRT female wizards and big boobs², a lot of it is development of the pectoral muscles and the contrast with a trim, muscular waistline, e.g. https://woman.thenest.com/can-strengthening-chest-muscles-increase-breast-size-7762.html
¹All hail Lucas, Lord of Darkness!
²It should be noted that not all of us think big boobs are really that sexy. Too much association with Holsteins https://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/Cow_female_black_white.jpg/220px-Cow_female_black_white.jpg at an impressionable age, I suppose.
The simple evolution of magic.
The most basic needs for humans are food, shelter and sex. Shelter could be anything from finding a dry and out of the wind spot to sleep to making fires to making cloths to building entire houses, and food&sex are similarily large.
Why they would look good:
Better food and shelter means indirectly better physical appearance
Having access to magically superior food and shelter will make you more wanted as a mate
Magically improving your physical body has multiple advantages: it helps you survive more, gather more food and build better shelters, helps you recover from dibilitating/ugly wounds and makes you look sexier to the others as you are better adapted to the environment.
As slightly metioned in the previous point, physical body manipulation can be used to heal yourself and others.
another advantage is that you can improve others around you, such as the mate you've chosen or you can help your children be strong, smart and sexy for their futures.
Magical manipulation of your physical body would have the most reasons for mages to pursue, so it's likely that if magic were real, physical body magic would be the largest field.
(aka the Rincewind/Harry Dresden style of wizardry)
To the general populous, a wizard is a being of unimaginable power. But in the grand scheme of things, wizards are actually fairly low on the totem pole of magical beings. They have just enough power to be attractive as batteries, but not enough to defend themselves from the more powerful beings who want to use them as such. So wizards become very good at running away.
And even the average, everyday wizard has got nothing on the research wizards who work in labs. When a chemist says "Oops", everyone with sense ducks behind a desk. When a wizard says "Oops", anyone who's still in the room 5 seconds later is a fool. And the really smart ones have cleared the building.
I can't explain the big bosoms, though.
Perhaps its just that they are hot and sexy compared the your typical common dirt farmer.
A wizard who knows to cast a spell to purify his drinking water will be worlds more healthy than one who gets theirs from the sewage and parasite infused runoff of the local stream. Commoners might get scarred from the many diseases and poxes they face. For wizards such ailments are unknown. For dirt farmers, toiling under the sun has brutal consequences on their skin, making it cracked and leathery. Wizards stay cool in the shade and have marvelous skin. Ever see the feet of a commoner? All calloused and gross from working the fields with primitive footwear. Wizards have nice sandals and boots. Scraggly and knotted hair? A simple spell will solve that. Mud stains (or worse) on your old clothes? Even an apprentice can keep their colorful silk ropes in top condition. Breasts need a boost... this old concoction the elves told you about will do the trick. And lets not forget bad breadth - a quick wave of the finger and the wizard's breath is now minty fresh.
Witches and wizards are extremely motivated individuals. They have to be or they wouldn't be any good at magic, because learning magic takes not only great intelligence and memory, but extreme dedication.
Most magicians' desires don't end with mastering magic. These narcissists are obsessed with proving their perfection and superiority over weaker people in all ways. Part of this obsession obviously includes obtaining physical superiority. And they are in luck; compared with mastering magic, getting fit is dead easy. All they have to do is regularly eat well and exercise, a small task next to the exhausting strain that is practicing magic.
I think Andrzej Sapkowski of Witcher fame gives one of the best explanations to this. They were not like that, female wizards were generally raised from noble lines. These noble lines found better use for their great looking women. Wizardy didn't improve the nobles situation because they were generally removed from the noble lineage creating their own (or terminating it as it were).
So mostly the women chosen for wizardy were in fact ugly. Since this was such a big stigma on the person chosen, more often than not one of the first thing they would do is to improve the situation.
Genetic correlation.
I recall hearing once a theory that the reason different dog breeds have such similar facial structures to each other compared to their shared canine ancestral stock was that they were bred for domestication, and the same genes for that facial structure were involved in tractability. The appearance (or that aspect of it) just sort of came along for the ride.
It just so happens that some of the same genes that control attractiveness (e.g. waist-to-hip ratio for women, chest-to-waist ration for men) are also related to magical ability.
The body needs to be fit to withstand the damage magic may caused, be it from other magicians or their own magic. Some magic systems even include martial arts. If they are supposed to fight against other people they need to be in their best shape. Most magicians also have access to healing magic which can keep their body in good condition. In most settings magic itself alters and enhances bodies of creatures that are infused with it or surrounded by it. That is one of the reasons for their longevity too.
In "The wheel of time" universe, magic is achieved by channeling an invisible force, not unlike "The Force" in Star Wars. One of the side effects of channeling is that it slows down ageing.
Similarly, you can say that magic has rejuvenating properties - the more more you do it, the fitter and better looking you become.
A lot of good answers, but one that nobody else seems to have gotten to yet: magic runs in families, and magicians have been able to find attractive mates for millennia. Therefore, they all have the genes of their attractive ancestors.
(Note: Only works for "magical races")
Most species evolve primarily for survival-related traits related to finding food or escaping/fighting off predators - strength, speed, durability, and so forth. In species where life is easy, where food is plentiful and there are no natural predators, sexual selection becomes much more prominent - which is why many island birds (for example) have exaggerated sexual traits, whether visual (the tails of birds of paradise) or otherwise (the impressive booming calls of the kakapo).
There is actually some (controversial) evidence that this has occurred in humans; our appearance has become more neotenous (child-like) since prehistoric times, and there are some theories that this is because as we relied less and less on our bodies for survival our sex appeal increased. This may be increasing further since the industrial revolution, though it is still too early to tell. While there is some overlap between physical fitness and sex appeal in humans, it is not quite as direct as many people believe; athletes (especially lifters) do not look like models, and fitness doesn't have a huge effect on the face in any event.
Wizards, by virtue of their magic, rely far less on being good at doing stuff physically than non-wizards (especially in a medieval world where most people do a lot of manual labor). Breeding among magical races will therefore place a higher emphasis on sexual traits which will leave its mark on the race over time.
