Let's say he gets hurt in the bowl game and all of a sudden he can no longer play in the NFL. Is Stanford going to pay him the full expected value of his lifetime NFL earnings? Obviously they aren't so why should he be so loyal to them if they aren't going to be just as loyal to him?
He has already given Stanford far more than he has taken from them. Setting himself and his family up for financial security is a far more important part of his life than playing one more game in a meaningless bowl game. And rightfully so.