They don't have time.
Go to college on a football scholarship and tell coach you want to major in mechanical engineering. See how long you stay on the team. If these guys aren't traveling to games, they're at practice. If they're not at practice, they're at the gym. Homework? Studying? Ain't nobody got time for that!
There's a reason athletes major in underwater basket weaving. There's not time for much else.
Even at the D1 level, only a few percent of players go pro.