Automation for flight is easier because there is a central authority controlling all (major) flights. Airspace and behavior therein is very tightly regulated, and the automation has a lot of dependencies upon extra-vehicular infrastructure (navigation aids, approach radio beacons, etc). There's also relatively few planes in the air at any given time. Scaling that up would not be trivial.

As for that Kittyhawk prototype - one prop failure at altitude and the pilot is dead. There's a reason they're testing over water at very low altitudes. Quadcopters have a very bad failure mode: they always crash. You want at least six motors, if not more, to avoid a crash every time a single motor fails. But, of course, more motors requires more battery power, which raises the weight, which makes it less maneuverable, and so forth.

From a pure "worst case failure mode" ground traffic is a better first automation target for moving small quantities of people.