Huge reason for that: It's called a collective bargaining agreement.
There is one in every pro sports league where the players are unionized. There isn't one in college sports. Yet.
If the players officially become university employees that would unionization easier I would imagine. (I have never been in a union and don’t know much about union laws).
I don’t know if the format of college sports would make unions feasible but we’ll see what happens. Unions have good and bad points but further discussion on that gets this moved to the political board.