Part of it is the US has a lot of land. Japan is one of the more extreme examples but they have ~8.7x people per unit of land. That makes a big difference on the kind of buildings and transportation you build.
One begs to ask the question, Why do Americans need to live so sprawled out? I think the difference is not just one of geography, it’s cultural. It makes sense for Japanese to live in close quarters and there are benefits and negatives for doing so.
Look at France, Germany, Belgium, UK Etc they’re allrelatively population dense countrirs with far more hopeless train networks.
Also, rail roads are not walkable, so they cannot be shared with pedestrians, so this limitation forces create two separate roads: one for pedestrians, bicycles, cars, etc., and second one for rail transport. Trams can be mixed with regular road traffic, but trams cannot use regular rail roads because of different construction of rail road.
Also, rail roads are prone to Single Point of Failure problems. To prevent SPF, multiple redundant routes must be built, with lot of expensive junctions between them.
Also, you must schedule your travel.
So, we need something which 1) can be shared with regular road traffic like trams, 2) can deliver loads of cargo efficiently like rail or metro, 3) can be connected to every house like regular road, 4) can be navigated by any car at any time of day or night, 5) be cheaper than regular road for road, cart, maintenance, recycling, 6) can provide power to carts, 7) can provide navigation and scheduling service to carts, 8) be with low delay (fast).
Regular road is 1,3,4,±8. Uber is 1,3,4,5,7,8. Aero-taxi is 1,3,4,±5,7,8. Trams is 1,2,6,7. Metro is 2,6,7. Rail is 2,5,6,7.
Edit: Tram-train is 1,2,5,6,7.
Actually, they can. https://en.wikipedia.org/wiki/Tram-train