Well in Davis' case, it wasn't by choice (leukemia).
Kinnick simply had much higher asperations than football (many teammates thought he would have become governor/senator/president had he not been killed), and recognized that football was just a game. He also lived in a time when college athletes were actually STUDENT-athletes. Players were much smarter back then than they are now.
And Ward chose to play in the NBA, and had a very good career there.