Well, the UN ultimately mostly reflects actual geopolitical reality. It's not actually a world government, there is no world government. It has the power nation-states choose to give it. The countries that wield vetoes? They also generally wield real "vetoes" IRL, ie., they've got nukes/massive militaries/economies. The formal legal veto they have in the UN merely reflects that if there was no UN, they'd have options on things they didn't like regardless. A basic point of the UN was to try to prevent WW3, and in that respect it did pretty well. For all the ideals, a lot of the core parts of it are pretty pragmatic about the limitations embodied by definition in anything "international". It seeks consensus and to avoid hot conflicts, and the former is pretty important to avoiding the latter.
Obviously it's not entirely without power of its own, particularly various kinds of soft power. But that soft power has sharp limits without hard power backing it, which is a very sticky wicket in most scenarios that make the news.
