Yes, China is about the same size as the US, but a route from Beijing to Kashgar is 23k feet up and 21 k feet down. A route from Washington DC to Los Angeles is about the same distance, but has 77k feet up and 77k feet down. A lot more elevation to work through. Much of the east side of that map is relatively flat. And there are bottlenecks getting to the west side, as there would be in the US, but the US has a lot more relative population on the west side than in China.

I'm sure that rail wins over flights in terms of efficiency for passenger miles traveled. However, you often have to travel more miles to get to your destination. A direct flight can save a whole lot of time. Flights are much faster, although security theater adds a lot of time[1]; but for longer routes, or routes with many stops or connections for rail, the single segment operation of a flight starts to win.

Also, China has the advantage of authoritarianism which allows it to more easily get right of way that's well aligned for HSR. While it was once easy to get land for railroads, when there were active land grants, now you'd need to do a lot of work to get well aligned land to add a rail line; where they're still active, existing rail right of ways through developed areas are fine enough for slower service, but expanding the right of way to better align the tracks is a lot of work. In theory, you could eminent domain, but that process is long and expensive.

[1] In a world where you think it's possible to have government intervene to expand passenger rail in the US, it's equally possible to have government intervene to streamline the security theater in flights; or they might add the same security theater to passenger rail.