Every time I've taken a taxi in a group, the conversation ends when we get in and the driver is suddenly a part of our conversation. It's weird to have them sit there listening to your discussion but not participating, and it would be weird for them to participate. Inevitably the conversation turns to the mundane, questions about whether it's a busy day, how often do you drive in this part of town, etc.
I got a Waymo with friends and the conversation never missed a beat as we got into the car, rode for 15 minutes, and got out.