The UN Security Council, as outlined in Article 39 of the UN Charter, has the ability to rule on the legality of the war, but has yet not been asked by any UN member nation to do so. The United States and the United Kingdom have veto power in the Security Council, so action by the Security Council is highly improbable even if the issue were to be raised.
No one cares and even if they did it can be vetoed.
Countries shouldn't be able to veto things about themselves. That's stupid.
Even if you remove the veto power, what exactly would you expect to happen?
Bush wasn't going to be arrested and put under an international court for the same reason Putin isn't going to be arrested for invading Ukraine. You can tell them "hand him over", they say "make me", and the only way to enforce the decision involves war, which no one wants to have.
The veto power is a problem, but it's not the main problem here.
The main problem is thinking the UN is supposed to be like this higher authority. That isn't and never was the intention, because it's impossible for it to be.