Because right now, in the eyes of the law, they are still not employees, although several recent decisions have opinions that openly muse that they are in fact employees.
The entire economics of college athletics relies on the fact that they are not employees. Other students get tuition, healthcare, etc. as part of recruitment packages, so that's above board. But once they start entering contracts specifically around their duties as athletes, that enters a whole new territory that we're not in. That's why payments are happening for NIL (name, image, and likeness)...not actually competing as an athlete.