The reason is very simple: the EU does not have the capabilities of defending itself against a Russian invasion, so you don't give Putin a reason to do so. If he does strike first, then it's an attack on NATO.

Unlike the USA, the EU is not some kind of United States of Europe, despite the word "union". In fact the only nation in the world that could take on the Russians is... well you guessed it.

The same thing keeps the UN (or did you mean NATO?) back. There is no defense against a nuclear attack which Putin has promised us, only retaliation. But hey, that's the cold war starting all over again.