I think the grandparent forgot to quality autonomous with safe. John Deere (My employer) has been doing autonomous tractors for more than 20 years, there have been videos of dogs driving tractors around while the farmer sips lemonade in a lawn chair. John Deere lawyers will use any legal trick they can to get them taken down though because it isn't safe to not have a human sitting in the tractor watching for trouble (many videos remain because there isn't much to go on). The tractor could kill someone and keep right on going with no care so it isn't safe. Likewise drone aircraft can go one for a while because presumably any other airplane in the area will avoid it. Boats can cross the ocean because odds are it will not encounter another boat (relative to the size of the ocean).

What is hard about self driving cars is we want our kids to be able to ride their bikes on the same streets those cars drive on. Limit self driving cars to just hiways with only other self driving cars and we could have had them 15 years ago and there would be near zero accidents. Dealing with human drivers (who might not be sober), kids, wild animals, and the like is hard. (I'm sure those in self driving cars have a much longer list of things things that are hard)