Fishing village isn't overpowered because of three related things: It doesn't draw cards, it doesn't have a buy, and to break the game with fishing village you need an inordinate amount of them, and there's only 10 in the supply. (Contrast a deck of 4 silvers vs. 8 fishing villages).
Vanilla village has less variance because it replaces itself, fishing village costs you a card, which makes it less feasible without draw-- without a trim deck and a source of draw, it's a lot harder to consistently play fishing village. Likewise, there's not much point to get a mega engine going if you don't have buys or some other source of extra cards (e.g. haggler/remodel). Also, you need 2 of them for the consistent benefit, doubling the amount you need as opposed to non-duration cards you can play every turn-- there just isn't enough in the supply to break the game, and it takes too long to buy enough to break the game.
Ok, compare to game-breaking fishing village (Let's call it Ninja Viking Fishing Village)
+2 Actions +1 card (+1 Action,+1 buy next turn) (On your turn, while this is in play, all cards cost 1 less)
What do you think Ninja Viking Fishing Village should cost?