You have to deal with players. A group of 18-22 year old young men who aren't paid are much easier to deal with than rich players, some of who just chase money around. Also, NFL defenses are much tougher than college defenses. Think about it. Only the top college players go into the NFL, so unline the NCAA, there are barely any glaring weaknesses on any professional defense (when compared to college defenses)
There are close games in college football, but there are far too many games like 62-0 (VA Tech over Florida A&M). The difference in talent level between the top teams and the bottom teams is huge. That's why the NFL is a lot more fun to watch (in my opinion) than NCAA football. Almost every game is competitive.
Or the 70-10 raping Tech hung on Nebraska. Boy am I proud of them. One of my friends was at the game, and he said Tech never punted that he remembered. If only someone could be those sucky Sooner-or laters. I hate Okla-homo.