I think an important difference is in the necessary reaction time. Situations in which a plane flying at altitude under automatic control suddenly requires a sub-second reaction by the pilots are extremely rare, in traffic they happen way more often.
Obstacles don't suddenly pop up in mid-air, and a lot of infrastructure makes sure other traffic is nicely separated at all times.
Thus a plane actually can do most of a flight automatically and it's okay if it has to fall back on not fully attentive humans in edge-cases, because there is some time for error-correction.
The car equivalent might be if highways had lanes separated by walls and cars could detect obstacles a few hundred meters away, then taking the hands of the wheel wouldn't be an issue. On real-world streets, you can't be as hands-off as you could be in a plane at 35,000 feet.
Obstacles don't suddenly pop up in mid-air, and a lot of infrastructure makes sure other traffic is nicely separated at all times.
Thus a plane actually can do most of a flight automatically and it's okay if it has to fall back on not fully attentive humans in edge-cases, because there is some time for error-correction.
The car equivalent might be if highways had lanes separated by walls and cars could detect obstacles a few hundred meters away, then taking the hands of the wheel wouldn't be an issue. On real-world streets, you can't be as hands-off as you could be in a plane at 35,000 feet.