Technology

It's a difficult comparison to make because planes are maintaining level flight or making smooth wide-arcing turns or gradual changes in altitude, not quickly responding to imminent obstacles and traffic. Even in an autoland situation, it's supposed to follow a gentle descent slope that's planned long in advance. This type of operation isn't really possible with cars, so they require a whole other set of considerations and techniques.

There's less stuff to hit in the air.

And even private aviation requires hundreds of hours of experience and deep understanding of physics and extensive training before even being allowed in the air on your own. Let alone to fly others that's a different training and license. Using those fancy “it flights itself” autopilots require several extra thousand hours of experience and specialized training, a commercial license and to be under the supervision and employment of an airline. Otherwise you are barely allowed to use the plane version of cruise control. Even after all that, you are still required to maintain your training with regular recertifications every few years, and a set of several hours of practice flight every year. Miss either condition and you lose your license.

And it requires way more training and attention from the operator because that way they can react quickly. Not so much for cars, especially on "autopilot".

No it's not at all, there's still a ton of work for the pilot and first officer despite autopilot