Its for the same reason most manga is set in a highschool (in no particular order):

1: It is a crutch to supplant lazy writing about what characters do the rest of the time. What do these people do when they're not in an action shot? Class! How do these people make a living? They don't (usually)! Don't even need to think about it. What kind of experiences do they have that might influence decisions? Stuff from school! What clothes do they wear? Uniforms!

2: It is a very marketable setting. Everyone (mostly) can relate to going to highschool, the same can not be said for university or any other path after that. It allows you to have a lot of young people as characters (even young adults don't feel too far removed) (also ) which are good for sales.

3: Pursuing dreams like major league sports after you're of age is broadly discouraged if you're not literally the best. This one is a bit harder to substantiate because I'm not from Japan, but when I went to study my non-stem major I got weird looks, I can't imagine what it would've been like to pursue mostly sports.