Why don't all countries simply dismantle their nuclear weapons if no-one will use them?

That's the whole purpose behind the nuclear disarmament treaties. There are currently 9 countries with nuclear weapons, and for the most part, only the U.S. and Russia have greatly reduced their nuclear capability. The others really don't need them either. It's more like a " don't mess with me " kind of thing, like when India and Pakistan tested their weapons on the same day...lol...on a more serious note, I would have hated to see someone like Osama bin Laden with an A bomb.