To add to this, aircraft autopilots have one huge advantage over terrestrial based autopilots: the lack of need to manage collision avoidance. This is handled by ATC in most cases, with the pilot only offering minimal backup (in the case of flying through clouds, no backup).

Any vehicular autopilot will need to reliably and consistently take care of collision avoidance. Any autopilot which wants to deem itself better than humans needs to do so much better than the equivalent human (and in a multitude of conditions, such as rain, snow, gravel, idiot drivers...).

And this doesn't even consider handover times. An aircraft autopilot could fail completely, and the pilot would still have on the order of minutes to recover (assuming this occurs mid-flight, at cruising altitude). A car autopilot failure would give seconds, in the best case.