A: The "problem" that government officials don't get to throw their weight around and break anarchic, bottom-up, emergent systems that don't kow-tow to them.
I doubt having the UN in charge is preferable in any way to having the US govt in charge.
For all its faults, the US is a democracy where people do have some influence on the political process. The UN is essentially run by ad-hoc coalitions of tyrants.
Actually I'm not a US citizen at all, and still I have a better chance of influencing a decision by the US government than most UN member nations' citizens have in their own countries.
A: The "problem" that government officials don't get to throw their weight around and break anarchic, bottom-up, emergent systems that don't kow-tow to them.