Where did you get this information? There are definitely people ready to respond to any requests from the car or passenger when they come up, but I've never read anything to indicate that each ride is actively monitored by a person.
And they're not drivers, they're supervisors. They don't take over driving, AFAICT they issue high-level commands like "safe to proceed", "take the right lane", et cetera.
I mean, this is true for Uber as well.