alt Hacker NewsRecover from upsets is the big thing. Maintaining flight level, speed, and heading while upside down isn’t acceptable.
Levels of safety are another consideration, car autopilot’s don’t use multiple levels of redundancy on everything because they can stop without falling out of the sky.
That's still massively simpler than making a self-driving car.
It's trivially easy to fly a plane in straight level flight, to the extent that you don't actually need any automation at all to do it. You simply trim the aircraft to fly in the attitude you want and over a reasonable timescale it will do just that.