Because they want to leave their school on a good note. Some kids actually enjoy their time in college. I mean Colt McCoy got injured in the BCS title game.
Playing in the BCS title game is not the same as playing in the Belk Bowl. No reason for RG3 to risk future dollars by trying to win the Alamo Bowl when that cat will be a high first round draft pick. National title game is a different story, though.
These kids don't even get financial bonuses for participating in postseason games. Sure, they might get free swag, but most of those guys sell that stuff for money once the bowl game is over.
Shoot, you have Illinois assistant coaches threatening not to coach in the Kraft Fight Hunger Bowl due to contract disputes, yet nobody is advising the student-athletes to think twice about playing in a meaningless bowl game and placing their potential NFL careers on the line.