On my alternate earth, elves originally evolved their phenotype (pointed ears) in the forests of Africa and migrated to other forest biomes afterward. After settling down they developed the racial differences we see in our world today. Darker skin near the equator, monolids in the east, etc. But all elves regardless of placement still share their trademark pointed ears with only minor changes such as size and length. Why would they still share this one trait after hundreds of thousands of years of separation?
If the ears don't affect the elves ability to reproduce/survive then the frequency of that trait in the population is not likely to change.
So simply put, they would all have the ears because there's no drive to evolve differently shaped ears.
Due to random mutation non-pointed ears may occur in individuals within this elf population. Given enough time, it is likely that you will see a minor population of non-pointy eared elves arise. Examples of this would be left-handed people and people with red hair.
We know that a population lost traits depending in many factors wich afect their chances to reproduce and be more succesfull in their environment.
Building a natural armor around your body, spikes and shells, is very expensive. If you remove the natural predators of this ecosystem, this traits wouldn't give a edge anymore and just become a burden for the creature. Making it disappear faster.
Easy Genetic adaptation
There is traits that don't require huge amounts of time to be produced, since the DNA change that is demanded to get it isn't that complex. This is deduced since some cave fish begin to lose sight way faster than expected.
For your elves, could be that this trait isn't critical, expensive or that the time that's needed to be forfeit needs to be longer.
There's two ways to lose a phenotype: natural selection and a genetic bottle-neck.
But the first thing I have to do is question your assertion that these separate populations have "the same" ears. It should be obvious to you that you can't claim the same shape and ignore "size and length"!!
So, what you're saying is that except for the characteristics you choose to ignore they haven't changed? Need I say more?
The most obvious reason to maintain a phenotype is because it has a significant benefit. If you take a look at rat/squirrel ears and compare them to deer ears, there's a range of shapes (together with placement on the skull, as well as skull shape and no doubt inner ear geometry). What we can potentially conclude is that there is little variation because their shape significantly helps them survive (more specifically, helps them reproduce).
This seems a bit implausible. You need to forget about time spans measured in years, for phenotype changes, you measure time in generations. If an elf lives 1000 years, then 100,000 years is only 100 generations; only very very beneficial or deleterious mutations will significantly impact the populations at those short times.
It gets even worse with sentient and technological species: their control of the environment dominates their physical phenotype. We (humans) are still evolving, and yet it isn't apparent to us. Are you sure that these various populations of elves haven't evolved different ears? There's more to ears than "a pointy tip". Wikipedia lists 15 visual characteristics of the human ear here: https://en.wikipedia.org/wiki/Auricle_(anatomy)#Structure, it's pretty implausible that in the analogous elf ear, that separate populations have maintained their uniformity across all of these phenotypes.
It may be more of the difference between our ability to recognize human faces (quite exceptional) and our ability to recognize the difference between, say, two crows. It's not that they don't have "obvious" differences, it's just that we don't notice them.
Elves might be an insular culture, seeing crossbreeding with round-eared species as a bad thing. This might be reinforced by an increase in deformities and so forth from such crossbreeding, such that they are called "monsters".
In such a society, where the pointiness of one's ears is a sign of genetic purity and likelihood of avoiding genetic problems in one's children, you would expect a selection force towards pointiness.
The existence of some round-ears in each generation - whether just as a random mutation, a dalliance with a human, or rape by a human hunting party - is unavoidable. But if these are treated as second-class citizens, then what elven lady would want to mate with them and doom her children to such a life?
Hundreds of thousands of years is fairly short in evolutionary time. If there's no strong pressure, it would be shocking if the ear shape did change. You may as well ask why no humans have pointed ears. After all, those pointed elvish ears are quite sexy. Very few elves would choose to have children with an ugly, round-eared, human-looking elf. It's a marvel that humans can hear at all with those weirdly shaped things that are far to flat to the sides of their heads.
The pointed shape allows elves to hear extraordinarily well. The pointed part catches sound and directs it to the lower ear. The ear can swivel to pinpoint the direction of the sound and hear it better. In other words, it works quite well as is, so there's little or no evolutionary pressure to change it. Perhaps the average ears are even a little bit longer than they were a few thousand years ago.
Evolution is not a defined cut and dry process, especially if there is no need for a specie to evolve (IE it maintains apex dominance).
As a world building creator you choose how you want flexible processes such as this to progress.
It is perfectly reasonable to say in this case: "they just didnt".
Who says that the Fae, as a magical race, are subject to the same rules of genetics and evolution as humans? And even if they are, they have such long lifespans that "evolution" would not be evident to a people who have lives that only last a blink of the eye in comparison.
You could justify the evolutive permanence of pointy ears all around the world if there's a good reason for the ears to have such points. Since, in your story, elves branched from the place where the dearth of their species happened to all over the world, and all share the pointy ears, there must be a reason for these ears to had evolved in the first place, differentiating elves and another species from a common ancestor. Hence, my suggestion:
Pointy ears as magic antennas: Many animals have antennas as probes to something, be them electrical fields or chemical gradients. Elves are always associated to magic, so why not let the tips of their ears to be antennas for magic fields? With such sensory organs you could nicely explain several mythological traits of elves, like their tendency to gather in places where magic is strong and to perform some simple magics.
This could be a reason for the elf speciation, when the Homo Elficus branched from our evolutionary tree (I'm supposing humans and elves are somewhat related, due to the very similar phenotypes). In the moment an individual was born with a magic sensory organ it could, then, benefit from this magic field, let it be for hunting, for gathering or for extending its lifespan. With time, elves became more and more sensitive to magic, to the point the could now control this magic field to their desires.
Other magic-related traits could also had evolved, like growth or regenerative factors which require magic, hence explaining their lifespan as well.
If they hybridize with humans a lot, it's predictable they will get more variation. Are pointy ears a dominant trait, or recessive, or something else? It isn't at all obvious how it would go, but more interbreeding would eventually result in more variation.
So they do not interbreed much. If round-ears are dominant, then halfbreeds could be killed at birth. Or considered human and thrown out of elf society.
On the assumption that the OP's elves are a different species, and not just a human racial group, the primary reason why a trait without other obvious utility becomes universal, and even exaggerated, is sex.
So I propose that elves have pointy ears because pointy ears are attractive to other elves. They are an erogenous zone as well as a visual attractant, and part of the act of copulation involves partners caressing each-other's pointy ears.
Without this sort of feature, other elves would look at such an individual and think - or say - 'Ew! Non-pointy ears! What a turn-off!' If humans are present in this world, this might account for speciation between humans and elves... effectively humans just aren't attractive to elves, having non-pointy ears. Maybe elves lack some other feature that humans like too... maybe male elves are too effeminate, and female elves too boyish to be really attractive to humans, and the ears just don't do it for humans the way they do it for other elves.