I'd guess that's a classic case of economies of scale, although inflated property costs and government corruption could also be playing a role. However, compare to China(wiki):

> "The HSR network reached just under 38,000 km (24,000 mi) in total length by the end of 2020. The HSR building boom continues with the HSR network set to reach 70,000 km (43,000 mi) in 2035."

At that scale, costs per km would be expected to drop off on a steep curve, due to economies of scale, distribution of initial investment costs over more km of rail, etc.