One reason is that it's already so expensive to operate a regular train, that the expense of having one employee (or even two) isn't as significant compared to a individual transportation. Paying the taxi driver is a significant part of the cost of a taxi trip. Paying the train operator isn't a significant part of the cost a train ride.
Edit: The article claims the opposite, and maybe that's true in NYC? I did find a breakdown of costs in Germany, for a municipal light rail service: operating the train is 1860 EUR per journey overall, paying the people operating the train (one operator, possibly one conductor) is 350 EUR of that. That ratio is smaller than I would've guessed, but it's not a majority.
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.
Edit: The article claims the opposite, and maybe that's true in NYC? I did find a breakdown of costs in Germany, for a municipal light rail service: operating the train is 1860 EUR per journey overall, paying the people operating the train (one operator, possibly one conductor) is 350 EUR of that. That ratio is smaller than I would've guessed, but it's not a majority.
https://www.wiwo.de/unternehmen/preisfrage-welche-kosten-ent...