It is always shocking to me when I land at an airport in the US and there no public transport available.
It is common for conversations about good local public transport to have a retort in some sub-thread about how big the US is, as if the feasibility of long distance travel affected the feasibility of other modes of local travel.
You mention the SF peninsula. When I first moved there, I lived in the westside of SF and had friends living in Sunnyvale. On a weekend, it took 4 hs to get to Mountain View (~40miles, at the time, checking now it seems like Caltrain weekend service might have improved, so the same trip would take about 2hs), and then had to be picked up by car to finish the rest of the trip. It was faster (~3:30hs, if more expensive) to go from Paris to Amsterdam (>300miles) by train.
Most people ARE interested in coast to coast travel. It is called flyover country for a reason.
There are a few exceptions like the Baltimore corridor, or the San Francisco peninsula, and these are in fact serviced by good trains.