You have guys who are being paid millions who are still voluntarily sitting this season out because they feel the risk exceeds that giant reward. But the bigger point is with the pro leagues, you have players associations to negotiate collective bargaining agreements to say players will stay in the bubble, what happens if they need to leave the bubble, and set out fines if players do that. Those agreements also set out standards of testing and when a player can come back if they test positive to mitigate the risk of spread.
College you have none of that to help guide behaviors or establish safety thresholds. And forgetting all of that, there is the wave where the NCAA is grasping the last straw that players are "amateurs" who should not be paid. When you have guys turning down millions with all of those procedures in place, it is kind of hard to keep arguing the amateurism sham if you force college athletes to play.