Flying a plane is easy. Autopilots have been a thing for a long time. Landing a plane is a bit tricky but doable. The value of human pilots though is in the case of malfunctions. That's the main problem. A failing safe-driving car has always the option of stopping on the side, calling home and lighting the warning lights.

A plane has to find a way of landing safely first. I am not versed enough in aeronautics to even begin to understand how hard it is to reach human-level in mayday situations.