Well, there's no world body to govern anything, let alone space travel. The UN is not authorized to make laws.
A [legitimate] government can only be formed with the consent of those it purports to govern, and right now nobody who can fly in space sees a need for it. After all, there aren't very many people in space, anyway. And the existing treaties cover it pretty well for now. In fact, I think some of those treaties go too far.