It makes sense why they wouldn't from a game theory standpoint. Why not shift liability? Waymo would too if they could set up such a structure in a way that makes sense. It is a little different for a cab where a 13 year old could call one on moms cellphone vs a car you buy outright and is registered to a licensed driver who pays for the insurance on it.
Still, my point is all this has nothing to do with the tech. It is all regulatory/legal checkers.