When I last was in the US reception (especially TM-Mobile) was pretty terrible outside of urban areas and we were mainly visiting national parks in California that were full of people…

I generally found reception to be better in the very sparsely populated areas in the Alps etc.

Also at least in some of Europe roaming between operators (for domestic users) is not really a thing and generally companies have independent subsidiaries in every country. Surely that would lead to higher operating costs to some extent?