[Michael Davis]

Because you are forgetting the times that your opponent will pay you off twice with a pocket pair or ace high. When he checks to you on the turn, he is announcing that he doesn't have top pair. If he does and decides to check it, hey, you got screwed. You also avoid giving him a free card on the turn when he does have outs to beat you. Checking behind on the turn is awful against a player who, say, will not bluff bet the river with ace high but will never pay off either.

There is no reason to take this to showdown every time. If you are checkraised on the turn, you can fold.

-Michael