BART is not a typical metro system in that it serves a lot of suburbs that have very little population density, and was mainly built as a commuter service to get people to downtown SF. So it was never going to have the kind of ridership the Tube has without massive upzoning and more infill stations. Comparing it to the Tube which mostly serves the city of London is not an apples to apples comparison. Look at the costs of building new rail infrastructure in London and it's comparable to here.

> If by "America" you mean NYC/SFBA then sure. You can see in your own link there's massive spread across the locales with some being cheaper than UK per km

What you're talking about in that link is the extension to San Jose, not day to day BART operations. That one does deserve criticism as they've made poor decisions like not doing cut/cover because NIMBYs in San Jose don't want any disruption to streets. So instead we are tunneling to the Earth first. Elsewhere in the world municipalities understand that it's worth temporary disruptions to roads to bring down costs, but of course America is unique and we have to learn these lessons ourselves.