(I am one of these people)
If you think about it, there really isn't much of a reason to, especially if you didn't go to the school you are rooting for. The level of competition and skill is much much higher in the pros, and most of us are fortunate enough to live geographically nearby pro sports teams. But there are still lots of people out there, who, for whatever reason, love NCAA football but don't watch the NFL.
I'm a diehard MSU fan, even though I didn't go there. I count the days until football season starts and follow them intently. Same deal with their basketball team. I don't follow the Pistons except very casually, and didn't even when they were good. I hardly follow the Lions either, but I think that's mostly because their games are blacked out a lot. I really have no justification