Computers work well in controlled environments, like a chessboard, a lab, a factory, maybe also a railway. They suck at uncontrolled environments like a forest or a dirt road.
The thing about controlled environments is that they are, well, controlled. They can be built specifically so that a computer can operate efficiently. You can automate the hell in a well-organized warehouse. You can’t automate much in a warehouse that looks like an unordered pile of stuff (but people are still able to work there, even if less efficiently).
A motorway is semi-controlled, which is why driver assistance technologies work so well. But it’s not controlled enough for unsupervised driverless vehicles. We can achieve autonomy on the motorway only if we make it with driverless vehicles in mind, with more rigid specifications. For example, when there are works on the road, we must place cones and signs in a driverless-friendly way.
Outside the motorway (or a very controlled road anyway) you can’t really get the driver out of the cabin.