And the market for taxis is much smaller than the market for selling cars to urban people. Robotaxis aren't going to replace car ownership, but that's the delusion propping up Tesla's stock.
Well, public transportation is a thing in a lot of cities. Autonomous vehicles can fulfill the role of buses/shuttles/people movers with more flexible routes and without having to cost of a driver. Maybe allow a bit more cargo for people who want to get their groceries home. But yeah, taxis don't seem like the optimal shape here.
45% of households nominally in New York City have a car and that is by far the lowest of any American city. In the ideal American city for not owning a car, almost half of the households own one anyway! This is about the same in Europe too. Pick a major European city and you'll probably find that between a third to a half of the households in that city own a vehicle, despite the famous European public transit and habit for walking and cycling that online anticar activists love to brag about.
In every other American city besides NYC, most households have a vehicle. And besides virtually every American city save one being built for cars, you also have to keep in mind that the "most people live in urban areas" statistic counts all the people living in the suburbs around cities, e.g. people who have chosen a lifestyle literally built around car ownership.
"percent of households that have at least one car" isn't a particularly informative metric here. If that car is tied up by a commuter then the other household members could still use transportation. And it also doesn't tell us how many households have more than one car and could get rid of a second one if public transportation provided more flexibility.
I don't think that autonomous buses would be as cost-effective as private vehicles, since you'd want someone to be on board for security on a shared vehicle even if they're not in the driver's seat. Labor is the most expensive component of public transit operating costs, at least in my area. That said, in low-density areas it's not uncommon to see just one or two people on a big bus that comes once every 40 minutes in the middle of a weekday, and I'm sure running autonomous taxis instead would provide better service at a lower cost, given the same subsidies.