This used to be true, but there's a feature currently in testing that will allow anyone (even someone coming from Google) to propose an edit to a post. This edit proposal can be approved either by a moderator or several users with sufficient rep. Hopefully this will make Stack Overflow more wiki-like and help solve the problem with answers getting out of date.
This sounds similar to how Discogs has implemented quality ratings (via votes) for arbitrarily edited content, in between full editorial control and open wiki.
