My understanding is that carriers here have to build a lot more towers per customer to cover everyone. Whenever you see a US coverage map that shows coverage in the middle of the continent, consider that there are a lot of towers out there in the middle of nowhere covering nobody.
Why?
They're trying to secure government contracts, which require a certain percentage of the country to be covered.
I suspect that part of the reason T-Mobile manages to be so much cheaper is because they're not trying to build useless towers in the middle of nowhere to secure a government contract - they're content with letting AT&T and Verizon compete for those while T-Mobile can more aggressively pursue businesses and individuals.
Also, Sprint is a joke. My understanding is that they aren't based out of the US, and they have little interest in competing in the US anymore. They're trying to sell what they do have to any of the other carriers, and they aren't interested in updating or maintaining what they do have.
So we really have only 3 real carriers, and 2 of them are more interested in government contracts.
Still, our cellular situation is much better than our broadband situation, where the providers each have regional monopolies and seem disinterested in competing with one another. As soon as there's an option to get the channels I want over the internet, I'm ditching Comcast and going cellular only.