It's almost certainly a hell of a lot easier to build a self-driving flying car than it is to build a self driving regular car. Regular cars have to follow roads, watch for people in unexpected places, adapt to road works, etc. Flying cars just need a rough direction to go in, and the ability to detect obstacles, with three dimensions to move around in to dodge them.
If that weren't the case, and we weren't able to create a self driving technology, I'd still question the logic that it's somehow more difficult to manually control something like this than it is a regular car. Why? What makes it harder?