Because their value is not strongly and equally protected. If you’re Mark Zuckerberg, Sean Parker, Bill Gates, etc. etc. and you have some amazing programming skills and endeavors that compete with college, you have the option of dropping out and still pursuing your passion.
Student Athletes don’t have that option. The NCAA took control over their value so you can’t be a standout athlete AND not go to college. That privilege is reserved for non-athletes, sorry.
This leads to some very unfortunate outcomes:
Ultimately this begs the question of “Why do early adult athletics have anything to do with academics?” The answer, “Because academic institutions control the value of college-aged athletes – if they want to play, they have to be in college.”