Yeah - this is why I find that argument very specious. Florida has a higher population density than France but there's no TGV equivalent running from Jacksonville to Miami
It's only one of the arguments. Florida has neither the political incentive nor the restrictions on air traffic to put pressure on government to build these things.
But the excuse is wrongly used, nobody is expecting bullet trains in Alaska or low populated places. So mostly people/politics are to blame and not terrain or density.
Please, I urge you to watch this. It explains thoroughly why the situation for railroads is very different between China and America.
tl;dw: America is more spread out, has better airline control, and less of a political incentive to expand public transport, than China.