I agree that the empty space in the western US distorts the population density, so just claiming sparse densities doesn't answer the question well. But isn't it obvious that there is are far fewer regional services in the US because Americans travel within the US more the Europeans travel within Europe? Lots more of the smartphone-carrying population travels from New York to California than from Spain to Poland.

Honestly, do you have any ideas about what causes these high prices other than density? The competition among the providers is pretty healthy, as far as I can tell.