I think the unspoken part of this argument is always that because this is an RPG, where the purpose of the game is to tell and interact with a cool story rather than to just kill monsters, the players do a lot of self regulating. Just because a Wizard or Druid can be build to be ridiculously powerful, doesn't mean that many players do it, as doing so would be boring. Unlike competitive games in which you are encouraged to make use of a broken build in order to win, you are instead discouraged from breaking your build in a tabletop RPG for the sake of enjoying the game more.
