#Magic is awesome because it lets you store and expend power
Did you know that you use exactly the same amount of energy for running and walking? Running just lets you get there faster. This is why pulleys and ramps and such are great - you get to spread physical effort over a larger amount of time, which makes it easier to move heavy loads.
Magic takes that to the limit. If I spend a week making skeleton warriors or enchanted swords or demon contracts, that's me storing up power. I can expend that power to make something happen in the world that I could not achieve with my flabby wizard body - topple a kingdom, raise a castle, crash an asteroid into the moon to get revenge on the Moon King once and for all.
This works great because "down time" is plentiful for everyone most of the time. If there's no rush, there is no advantage in running somewhere over walking, or carrying a heavy load straight up when you could use a ramp.
#Martial arts typically cannot do this
Your kung-fu warrior is generally going to be reactive rather than active. An enemy? Run over there and punch it. It is now dead...and until you see another thing that needs punching, your punches are useless. You can't spend the next five minutes concentrating your muscle power into a really awesome punch.
This is not only a problem of power, but also one of versatility. The wizard is able to store power, so he can have stored a whole bunch of different things (which most magic systems allow, because magic is magic and so can do anything). Any ability the kung fu master needs to have, he needs to have all the time, so he typically has fewer things going on.
#You can nerf the casters
The ability to store power is less useful in two cases. Case one: there is no opportunity to store the power. If it takes a week to complete the ritual of whatever, but you need the results of that ritual every day, the usefulness of the ritual rapidly diminishes, no matter the quality of its results. Case two: There are harsh limits on how long such power may be stored. If a necromancer can only have one skeleton warrior at a time (or only order one skeleton warrior to move at a time) then the skeleton warriors aren't terribly useful. If a wizard makes a magic sword, but the sword loses its magic in the next ten minutes, it's not as good as just being skilled with your regular sword.
#You can let martial artists store power too
You only need a small amount of fantasy to explain how they can do this.
- Meditation: By centering their minds and souls, your kung-fu guys can improve their abilities. Maybe they can force their own bodies to produce adrenaline and pain-blocking chemicals, allowing them to push their bodies to the limit. Maybe they channel the might of their ancestors. Maybe they do hella drugs. Whatever the reason, a small amount of prep time can give you a superman that can rival a magic guy any day.
- Intense planning: Like, really intense. Bordering on precognition intense. Sherlock Holmes meets MacGyver intense. By studying things like how an arrow flies, the warrior can predict where arrows from his enemies might land, and just not be there. A utility belt is a must, but you can make do with some pocket sand in a pinch.
- Allies: A very common structure of many folk tales is this one - the hero goes on a quest, during the quest he helps some magic talking animals, in the end he's in a bind but the magic talking animals come help him. Moral of the story: why punch people when you can contract the services of Enormous Bear 4 Hire Ltd. at a reasonable price?