@Luccajan basically the UN is a forum for dialogue and we need the big players to be part of it.
If they don't get veto on the security council they will have a tantrum and leave, which will benefit no one.
The superpowers already flout international law when they really want to, because there is nothing the rest of us can do to stop them, but it would probably be far worse if they weren't even part of the UN.