I believe playing time is at the heart of this. So many of these players go through AAU being told how great they are. Then they get to college and find out that everyone is that great. Or the coach that recruited them leaves. Or they find that the school they selected just isn't the right fit.
I should mention that scholarships are one-year deals, not four years. Sometimes, particularly in football, a coach overrecruits, expecting a certain number of players not to return. When he has too many players, he just lets certain players know they don't have a ride anymore.
I don't really have a problem with players transferring. It would be nice for programs to develop an identity over 3 or 4 years, but that's just not the reality today.