“It is not obvious to me why state governments in the US are not able to “tidy up” local government in the way that the UK has (even though I think the UK could do more). I can see that in a federation, this should be a state issue rather than a federal one, but it seems like it should be within the states’ power to remedy. I could imagine that it is just politics that makes such things impossible in such a divided political system, but if there were any legal reason that a state government that was so inclined could not do this, I would argue that this would be an undesirable restriction.”

Many states are influenced by the state constitutional theory of “municipal home rule” in which local governments are recognized to have an inherent authority emanating from the people rather than being mere creations of the state government. In this constitutional framework, both the state government and local governments are seen as creatures of the state constitution, with seperated powers, which can only be amended by a higher constitutional power (usually a vote of the people or a constituional assembly). State governments do retain varying abilities to regulate municipalities when there is a strong interest in having statewide uniformity. Although only 19 states have officially recognized this theory, (including many of the more populous states), it likely has an influence on politics in other states. Additionally, many states have state constituional requirements for changes in municipal boundaries to be approved by the voters of the affected areas. As discussed in my previous comment, this often leads to self-interested refusal by subareas to combine into more organic agglomerations for the better of the whole area.

Politically, a state government interfering in local government affairs is seen as upsetting the natural order of things. Americans have a general distrust in government which I believe grows with each level. So, Americans are more likely to side with their local government in a political dispute with their state government. Additionally, state legislators are often former municipal council members (and the political machines which elect them are often tied up in local government) making them unlikely to support changes opposed by their former municipal colleagues.

You are correct that the national federal government has almost no involvement in municipal formation/boundaries, leading to a seperate system in each of the states.

My takeaway is that in any federal or quasi-federal system (if we think of home rule municipalities as having a quasi-federal relationship with their state governments) it is important to have a regular process with strong constituional mandate to rebalance the constituent units. Or else population changes will eventually lead to arbitrary political boundaries and imbalances which undermine the functioning of a federal system. I think that it’s unlikely that the constituent units, through their governments or through referendums, will on their own resolve these issues in a way that is beneficial to the whole.