Because Kofi is a one-world-government communist, and as long as it's government and driven by taxpayer dollars, for him bigger is better.
If the UN were a one-world govt, in would be able to act in certain cases like Sudan or Iraq. The fact that soverignty is sacronsant even in cases where a country has turned on its own people is one of the problems, not its centralization (of which there really is none).