I was thinking that wheeled locomotion is still much easier than legged locomotion, artificial arms, grasping and fine motor manipulation. Robots still can't compare to a dexterous human, but a SDC is arguably a good driver.

Roads are a fairly structured and rule-based environment already designed with machines operating on them in mind. And those aspects that are not designed for the physical properties of cars are still designed to be easily perceptible by the human operators, e.g. signs, lane markings.

That can't be said for many other environments in which autonomous robots would have to operate.

There are literally millions of Roomba cleaner robots sold. They are autonomous.

Simple, but autonomous.