Removing the driver from a taxi doesn’t bring down costs that much. Self driving cars aren’t going to change the uber/taxi model at a fundamental level.
They have a finite fleet that they need to deploy. Urban centers mean that fleet utilization is high, and relatively less time and miles are spent driving with no one on board. In rural areas with little demand they will sit empty or have to drive empty for many more miles to their pickups. It just isn’t profitable to use your fleet that way no matter what you do.