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)

Some of the autonomous boats know to keep away from the bigger ships, the ones with transponders. Some airborne drones will react to TCAS alerts (Traffic Collision Avoidance System) and change course/altitude in the same manner as a human pilot would. I would assume that the largest military drones are capable of flying home, or at least back to friendly territory, if they are disconnected from controllers.

There are two hard parts of hazard avoidance: identifying the hazard in the first place, and dealing with large numbers of different hazards moving around. Transponders, TCAS, and maps solve the first problem well enough for boards and airplanes.(there are time where they fail but they are rare enough to ignore). Except at ports/airports traffic is generally light as well meaning drones and boats only need to worry about a small number of hazards which makes this easier.

Autonomous cars are hard because children, and wildlife do not have transponders, and worse have a habit of jumping out from cover right into your path. Killing children is obviously bad. Even though who morally would argue that wildlife don't matter still do not like the damage hitting the wildlife causes to the car.

The parent did specify land vehicles.