Part of your problem is that you are forgetting that most of the time people aren't just going all-in on the flop.

So you really aren't getting 1/3 on the flop bets to call. Instead you are getting 1/6 to hit by the turn and then another 1/6 to hit the river if you don't hit the turn. But by the turn you are often going to have to pony up some more.