And how many people regularly commute across the United States? The absolute majority of the journeys people make regularly is still quite short, so why not start by optimizing for them? Then continue with building high speed intercity connections between the urban areas with <500 km distance to create valuable alternatives to being stuck in the highway traffic and dealing with the airport security nonsense.
Why do you always think that you need to reinvent a tried and true solutions that have been proven to work across the world?
Also even if you're worried about going cross-country, that requires what, two fast lines between the coasts to get good routes? So even though those areas don't have many people per square mile, passing through would need an extremely small amount of track per square mile.
Why do you always think that you need to reinvent a tried and true solutions that have been proven to work across the world?