Due to the complexity and diversity in economic, cultural, and social value networks.
For example, the approach which is working for Modesto will probably not work for San Francisco.
That has less to do with the size of the US but everything to do with the lack of size in the US. We make it impossible to do things by making each city small independent, and having a lack of unity.
Our government is not more complex than Finland's because we have more people, it's because we chose to make it inefficient and complex.
Removing local cities' power to be different for the sake of complexity would solve the issue quickly. If the Bay Area had a regional government rather than tiny fiefdoms devoted to allowing wealthy people to extract the maximum economic value from shared business interests, while willing away their own tax dollars in tiny enclaves that are protected by minimum lot sizes and apartment bans, not only would we have far less homelessness to begin with, but we could solve the leftover homelessness much better, refuse crime and poverty, and have a far better functioning society.
Why do you think a regional government would be any more altruistic and charitable than a city government? I've seen a regional governmental (a metropolitan council like you suggest) that covers multiple cities in a metro area that have done nothing but squander money to justify their own existence. It got so bad that they ended up getting their powers curtailed by the state.
Everything else you mention is just wishful thinking that could be applied to any government regardless of size or scope.
Due to the complexity and diversity in economic, cultural, and social value networks. For example, the approach which is working for Modesto will probably not work for San Francisco.