That is mostly a zoning issue, have you seen the density around Tube stations? Compare that to the density around half of the BART stations which are big parking lots surrounded by single family houses. Of course it's cheaper to run a transit system in a city with twice the population density and population in the metro area.
Costs are an America issue, not a BART issue: https://transitcosts.com/new-data/
BART is one of the most cost efficient systems in the US: https://www.reddit.com/r/transit/comments/1d27dvo/us_cost_pe.... It's so efficient that pre-pandemic it got the majority of its funding through fares, not taxes.
By the way it costs exorbitant amounts to build highways too and you don't see people criticizing all of our highways around the area do you.
So quite frankly you don't know what you're talking about.