[cryptonector]

Because the U.S. is a) much less densely populated, b) is very spread out, c) leading to very long distances between cities (which makes rail expensive), but d) very homogeneous in culture and language, so e) families and companies spread out a lot, f) which means train travel is not really practical, but also g) many of these short-haul flights are connections to/from long-haul flights and there's no way we'd take a several hour train ride to then go to an airport, to then take a long-haul flight to then do the whole thing again.