The same reason you say you'll bet (there are 8 bets already in the pot) is the same reason your opponents will call. You have a better chance of cutting down the field if you wait for the turn. If a blank comes and it is checked to you, you have the best hand and can bet, or you can raise hoping to see a free showdown.
But if a heart or a straight card hits, you will have saved a bet. With four people in the pot, you have to give them credit for something, even if they are as bad as you say. You're looking at winning a small pot or losing a big one here.
Anyway, these decisions are easier with a reraise preflop, imo.