College football is a fickle sport for a whole host of reasons, but maybe one of the biggest is the very finite time that players have to try and make an impact on their teams. Unless you're an immediate superstar, there's a good chance that most of your career is spent riding the bench, just waiting for someone to be dumb enough or injured enough to give you a chance at glory.
Or, alternatively, you actually are really good, but just stuck behind a whole host of even better players on the depth chart. This is the kind of situation that comes up a lot at schools like Ohio State, because the amount of talent means that it can be next to impossible for a truly talented player to make an impact in the long run.