How about... no one makes taxis, companies make cars, most of which are bought by regular people who for obvious reasons want a back seat. So that's what taxi-drivers buy.

That's not entirely true. The TX4 (London Black Cab) is a purpose-built taxi in use in London and many cities throughout the world. The Toyota Comfort and Nissan Crew are also primarily designed as taxis. Even the venerable Crown Vic which was not primarily designed as a taxi (as far as I know) but has a factory taxi configuration which beefed up suspension, interior dividers, etc.

I could well be as you seem to suggest that most cab rides are two or fewer passengers, so therefore a Robotaxi could sacrifice two seats and two doors in exchange for lower production costs. If you have a group of three you just take two cabs.

But I'm skeptical this is happening in the near future. Tesla doesn't even appear to be the leader in autonomous driving, and even the leaders don't appear to be very close.