Look at a big box store, for instance - most of the land is parking, not the store itself. A freeway interchange can take as much land as en entire neighborhood. Meanwhile you can have tens of thousands of people in a city centre all within walking distance of one another. But you can't do that if you pave over most of the land.
Also, it's illegal in most of the US to build infrastructure that isn't suited to an auto-first lifestyle, mostly through parking minimums but also Level-of-service considerations that force large, wide streets.
In California, SB827 just died a sad death. It would have allowed buildings up to 5 stories near high-frequency transit stops. Now those people who would have lived in those buildings will have to live farther from transit, so they'll drive. They'll need parking too, so we'll need to use land that would have gone to homes/businesses/etc for their parking. Also gas stations, more road capacity, etc. The homes they end up living in will be harder to reasonably serve with transit, because of course they're farther apart.
Yet old neighborhoods near cities built _before_ those rules are often very expensive, suggesting that more people would like to live in them than are able to. The first thing that comes to mind is the nicer areas of Berkeley, or South Park, San Diego, for instance. It's funny you note railways though because they were largely the original enablers of sprawl (though I have no problem with railway-served suburbs! the problem is largely that we got rid of the rail service).
IIRC, that's ahistorical and getting the causation backwards. American cities used to be denser with more public transit (e.g. trollies) before cars became common. The reason cities are spread out is cars enabled the residents to spread out, since that's what they wanted to do. That established a new pattern of development that makes density more difficult.
Those things that make driving more pleasant also make walking less pleasant (walking next to Wilshire blvd in LA isn't exactly pleasant, for instance), so more people drive, so more people demand auto-oriented infrastructure, and so on.
At some point in our nation's history (namely right after WW2), it was decided that cars, suburbia, and the Interstate highway system was the way forward, and not dense, closely packed cities + public transport + more railways.
Note that we might not think about this now, but cities before WW2 were often disease-ridden, filthy places. The antibiotic revolution did not really come into full swing until after the war. There were reasons people made those choices back then, and some of them we have forgotten since the problems they faced were mitigated.
We're now revisiting whether that choice has outlived its usefulness, but if something took 50 years to build, you can bet replacing it will take just as long.
China never had the luxury of making that choice during its development; it was either public transit or nothing.