China has invested into high-speed rail for the same reason that Japan, France and Spain did: the big cities tend to be in the "sweet spot" distance of 250-750 km from one another, which is very comfortable to ride in a high-speed train.

Distances over 1000 km are already favorable for passenger jets. Even in today's Western Europe, there just aren't that many really long distance trains (by which I mean something like Amsterdam-Barcelona or Copenhagen-Milan).