It turns out the expensive part was not the digging. It was moving undisclosed utility lines and building the stations. Moving undisclosed utility lines is almost singlehandedly the reason the regional connector and Crenshaw lines are over budget.
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.
As for LA, you've had ample opportunity to expand your subway system over the past decades to solve this problem, it's just never actually been done. It's not clear how Boring Company changes anything. I would love to be wrong about this, but I'm not seeing it.