This ignores the massive value SDC's can provide to the blind, disabled, elderly, and children, all of whom may not be able to hop on a bike or scooter.
I did mention taxis. The question would then be, why an SDC as opposed to a taxi? In the example you give a driver would be of more use.
Because a car your own means your stuff is in it. If you use it somewhat regularly it is cheaper in the long run to own your own SDC over a taxi (even assuming automation), if only because you get to decide when to clean it.