[inglor_cz]

"Given that we can't figure out how to build subterranean cities at scale on earth"

Can't we, or there just isn't any economic case for it?

Cold War militaries were certainly capable of building massive buried structures when needed:

https://en.wikipedia.org/wiki/%C5%BDeljava_Air_Base

https://de.wikipedia.org/wiki/Proto_(Bunker)