They do it because they have been in this game long enough and know that the risk is not worth it in many occasions. I think weighing the entire situation of how the game is playing out, the distance involved and even the opposing defense your facing all plays a part. If I'm up by 3 late in the game and faced with a 4th and 2 from my own 40 I'm going to punt because I know damn well if we fail I just handed the game to the opposing. Using some avg where they take poor defensive teams along with the top defenses in a class room setting is all well and fine on game day when you are facing a top defense who are very capable of winning those short yardage battles is foolish. There is a big difference between a Miami defense and that of a 49er defense.