Although they did also put it mainly on flat land. Some of our most promising city pairs with high traffic and strong local support for such a project are unfortunately in or separated by mountainous areas: LA-SF, Seattle-Portland, Atlanta-DC, etc.
From the FT article posted elsewhere in this discussion:
According to state media reports, the government spent $17bn (€12bn, £11bn) on the Harmony express line’s construction over 4½ years. Wuhan invested $2.4bn in a new French-designed train station, which boasts 20 tracks and 11 platforms. Officials this weekend declined to confirm project costs.
One reason for the enormous construction outlay for the Harmony express was difficult terrain, especially in the poor mountainous areas of Guangdong and Hunan provinces. The train travels along 713km of elevated tracks and tunnels, accounting for about 70 per cent of its length.
So you're argument may not really hold water. I think the big reason for this not happening in the US, is capital costs, the lobbying efforts of airline and other affected industries and NIMBY mentality.