If by "almost everywhere" you mean Europe or Japan, please compare population density. If most of the US had the population density of Northern Europe, or US northeast, or Silicon Valley, it would have similar almost-omnipresent coverage.
US West is more like Siberia, or Canada 100 miles north from the border. There are not enough subscribers to economically cover with GSM / LTE, except in cities and along major roads.
I'm talking about places like India or even Myanmar. To be fair India has probably a much greater population density than the US so maybe you are correct.
