Driverless: the company needs an operation, real estate, and staff comparable to a big auto rental company to do all that. Plus the engineering and technical staff required for autonomy.
Even with startups doing autonomous shuttle buses, which works at low speed, nobody is making money in that space. It's all demos.
One additional factor seems to be that Uber drivers tend to underestimate the depreciation of their cars, so they effectively subsidize what otherwise would be Uber's capital cost.
And THAT's something you can't get a robot to do.
I recently listened to a podcast episode of the Autonocast[1], where they interviewed a Harvard Researcher who claimed the economics of Robotaxis just don't work. Very interesting listen.
[1] http://www.autonocast.com/blog/2020/3/11/177-ashley-nunes-on...
Early robotaxis will have tight geofences. Makes them particularly bad competitors to poorly paid people who are willing to drive wherever.