I have no first hand knowledge here, but thinking from first principles. From Robotaxi fleet perspective you want autonomous maintenance, cleaning, charging, and lowest cost. From Robotaxi user perspective you want climate / music / entertaiment / safety. So the idea for robot taxi is that it should be better than model 3 in some or all of those dimensions.

Now speculating for the moment, from Elon perspective you probably want things to be more cyberpunky as that is how future looked in his childhood and he is trying to build it. Also, engineers / designers were likely mandated to handle all of the maintenance and possibly production by Tesla Bots.