Wendover had a good video about China's rail network a while ago. [1]
Major points are:
1. China is willing to build loves that aren't self sustainable - whereas the USA isn't (and the US doesn't factor in the social benefits)
2. China's airspace is mostly military - leading to only small flight corridors and therefore significant delays for flights - making trains more reliable
3. China is willing to make train lines that make no sense beyond connecting areas that are not as friendly to the government to areas that are in the hope of increasing government support in those areas