It depends on whether you calculate it as including the amortized fixed costs (e.g. the cost of building the tunnels) or the incremental cost (what does it cost to have one additional passenger). If it's the first one then the cost is way higher, but then you'd have to do the same thing in the other case and include the cost of building roads etc.

However, fixed costs are better funded by general taxes than by usage fees because otherwise you pay a huge fixed cost to build something with a low incremental usage cost and then under-utilize it because recovering the sunk cost through fares causes high fares which deters uses whose value exceeds the incremental cost.

Meanwhile human labor is a significant proportion of the incremental cost, when you have humans doing things per-trip that could reasonably be automated.

Self driving cars do exist and operate every single day.

honestly I would have thought it was an artifact of labor unions -- at least here in the states.