autopilots do a bit more than maintain course, they can basically execute the entire flight-plan in between takeoff & landing, which includes plenty of heading adjustments. They also increasingly do a fair bit of heavy lifting on landings as well.

A plane is much easier to navigate because there aren't things like lane-dividers floating about. That's handy, but alas it does not apply to driving a car.

Just because an in-air navigation aid that allows for essentially pilot-free navigation deserving of the name "autopilot" happens to be relatively trivial to build, does not mean any navigation aid of similar complexity for other modes of transport should be called autopilot simply because they're at least as complex. If you're not automatically piloting, the name autopilot is pretty disingenuous, is it not?

Also, planes have a bunch of other navigation aids, including collision avoidance systems and ground-based air traffic control, and pretty obvious stuff like predictable flight paths and huge safety margins between nearby aircraft. An autopilot for an airplane works in that context. I kind of doubt it would work if there were thousands of planes nearby, many of which are on paths separated by distances the vehicles in question cover in tiny fractions of a second, and some of which follow almost invisible rather unpredictable paths (that drunk guy without lights over there...). Context matters.

I think what I'm taking issue with isn't anything about comparing them to self driving cars, it's that you characterize them as "trivial to build". They are not. They coordinate a significant number of life-critical systems, react to minute perturbations in the air stream with microsecond precision, and do so with such a high level of precision that human pilots are often forbidden from overriding them outside of specific situations. I don't know what gave you the impression that these systems were basic or trivial, but they have been at the cutting edge of systems automation for decades.

Compared to a self-driving car system, they are trivial. Your assertion that "they have been at the cutting edge of systems automation for decades" kind of indicates that - this stuff is simple enough to make that it was feasible to do even that long ago.

Trivial is good mind you because it actually works, and works reliably. The same is not yet clear of self-driving cars.

Well my kap 140 sure doesn't do any of that stuff.