There is nothing to achieve by making such a bet. Either nothing happens (he folds) or you blow your brains out (he calls with a better hands). It is a bit like playing Russian Roulette. If you did that, your opponent would simply be folding the missed draws and anything worse and calling only when he has you beat. Therefore, it would make no sense for you to bet the river with medium strength hands. By the time you reach the river, having bet both the flop and the turn, and having been called, Villain will hold a high concentration of made hands above a certain strength and no air apart from maybe missed draws.
When you bet again on the turn, Villain is again supposed to fold the weakest his weakest hands, leading to a further contraction in his range. This makes his range narrower and stronger than it was previously. When you bet the flop, you force your opponent to fold some hands.
