Imagine deciding after your freshman year that you wanted to study at another school, but your current school wouldn't allow you do so, and so literally no other university would admit you.

In the case of athletes, you know there are plenty of guys who never got their chance to play just because they weren't a great fit with the coach or the system, or there was someone better ahead who was sucking all the oxygen out of the room. Some of those guys, had they been allowed to transfer, would have ended up going pro.