Or put another way, if you don't want or need a card in your deck, chances are good that the other players don't want or need it either. They don't want Estates, Coppers, or Curses. And by the time you're turning good cards into Provinces, they may never draw their new copy of that card before the game ends. So really it's only a bonus for them if you're turning good cards into better cards in the middle of the game. And Remodel/Expand aren't really great at doing that anyway.