It's much easier to build rail in places that weren't designed around cars. Even in rural China people live in condos and apartments with parks between. This helps with NIMBYism and combined with the already large amount of green space left in Chinese cities such systems can be built with the only real concern being the engineering itself. But China is also in a good position for that, as their workforce is incredibly well educated with more engineering talent than they can even fully employ domestically. All that PLUS the political will of a single party state meant it was a very different situation than California.

And that's before you even consider ridership, where even the best possible SF to LA route would still pretty much require you to get a car or taxi once you get to LA (because LA was basically torn down and redesigned for cars).