Re: supply, demand, the need for humans...
Ultimately, the driving part of the job is only a bit. Taxi drivers also shift luggage. And possibly wheel chairs.
A driverless car could still be serviced, it'd just be another amenity to pay extra for, in the same way as a posh restaurant will have someone to take your coat while at Nando's you sort it out for yourself.
More than likely it'd be easier to order a serviced cab via an app than walking along the taxi rank to find a driver willing to leave his cab.