The UN is not a world government, does not maintain a military force, and prints no currency. They have their own stamps
but nobody uses them, apart from collectors.
The UN is an expression
of the will of the allies to avoid another world war after WW2. Just like the League of Nations was the expression
of the will of the allies to avoid another world war after WW1. It's a tool
for diplomacy, and like any tool it has no independent will or force. If its member nations want to use it, it's useful, and if they don't want to use it, it's useless...
Recently it's been undermined by both the US and Russia. W's Gulf war undermined fundamenal idea that no nation can attack another nation without an explicit UNSC mandate. And when Putin attacks Ukraine, everybody goes "Oh well, who cares?"... The whole idea of collective security that underpins the UN is now going down the drain. The UN is spiraling towards its death, like the League of Nations before it, because some of its main sponsors (the US, the UK, Russia) have abandonned it.
Until WW3, that is. And until the next attempt to promote peace, cooperation and diplomacy through such a global thingy... What it's gona be called next time? The Club of Countries?