And subways have relatively few surface interfaces compared to what you'd need to build to move the same amount of people in cars. Come to Manhattan and look at the massive, multi-block entrances to the Lincoln Tunnel, Holland Tunnel, Midtown Tunnel, etc. You're talking billions worth in real estate acquisition costs, demolition, and construction just to build a new tunnel entrance in the city, completely ignoring the tunnel cost itself.

And these tunnels move fewer people per day than the most-used subway stations, which are easier to integrate into the urban environment because you only need to build stairways and a couple elevators, not massive ramps and road connections for vehicles.