I agree with you (and importantly you can't make a subway political football the way you can make a bus lane), but my experience doing transit advocacy points otherwise. Americans in dense areas are feeling the HCOL pinch and are not very willing to float extra taxes to fund transit expansion.

IMO it comes back to the fact that Americans are just not willing to accept change of any kind right now. The economy feels too shaky, the electorate too divided (even within states and municipalities), and there's too little faith in government to architect the kind of change you'd need to build subways, underground metros, or even BRT. We need a larger feeling of unity even at a state level to make the changes necessary, which is why municipalities continue to do bare minimum maintenance of roadways and pretty much nothing else. The last big set of constriction in dense urban areas was funded by the Obama stimulus from the GFC which was passed 17 years ago.