To be fair, Waymo claims to not record or transmit audio without you either manually engaging such (by requesting support), or a very unambiguous announcement (presumably when the car gets into some sort of emergency situation). And lying about that claim would probably run afoul of California's 2 party consent law. So still a step up in privacy versus having someone in the car listening in on your conversation.
That said, even if they were listening to you, there's a lot of things that are completely inconsequential from a perspective of an anonymous call center employee far away listening in on, that I probably wouldn't want to talk about in front of a taxi driver.
I know this is somewhat besides the point of the discussion, but.. many Ubers have recording devices inside the car too. The drivers have gotten savvy and protect themselves from false claims or even harassment.
That said, even if they were listening to you, there's a lot of things that are completely inconsequential from a perspective of an anonymous call center employee far away listening in on, that I probably wouldn't want to talk about in front of a taxi driver.