When a highly-drafted player stays @ Bama/Clemson until his Senior year, it's typically because he didn't get much playing time (until Jr/Sr year). He rode the bench, because he was behind other highly-drafted players.

In other words, you're not going to find too many examples where somebody is All-SEC as Soph/Junior, and then stays for all four years.

As for Miami in their recent glory days (2000-2002????), some of those guys did stay. But still, you had the same issue of future NFL players stuck behind other NFL players (Frank Gore, Clinton Portis, Willis McGahee). Also, back then, the # of players leaving early wasn't even close to today's number. Apples to oranges.