let's say you continuation bet only when you flop TPGK or better. an observant opponent is going to eventually realize you have a very low continuation betting frequency and that when he calls your continuation bets and gets to showdown that you never show down a weak hand. then, he adjusts his game by folding to your continuation bets when he doesn't have a decent draw or a made hand that is somewhere higher than the bottom of your c-betting range, and taking all the small pots away when you check to him. since this range contains only legitimate made hands, it is highly unbalanced and your opponent has an easy time adjusting to and exploiting you.