Trains can be automated because they mostly operate in one dimensional space with minimal interruptions.

Automobiles operate in two dimensional space with lots of interruptions, but the fail state is to stop at the end of a two dimensional trajectory. We still haven’t fully sorted this one, so cars are limited to automation in conditions that are reasonably well defined.

Airplanes operate in three dimensional space with no marked paths and all sorts of unforeseen choices, from shifting winds and weather to fixed obstructions to wildlife and other airborne objects.

And yet despite that, much of what a modern passenger jet does is automated — autopilot navigates while at cruising altitude, and ATC manages landings.

But then there’s takeoffs and the many many things that can happen in an unexpected way in three dimensional space.