Self-flying airplanes are closer to reality than self-driving cars. Not that either are close to reality IMHO. Commercial airplanes operate largely in a controlled environment, under control from ATC, and with other commercial airplanes which follow the 'rules of the sky'. Yes I know there are lots of exceptions and emergencies/failures (largely solved without drama by experienced, professional human crews). But for the most part, the in-flight behavior of commercial airplanes is fairly predictable.
Self-driving cars have to deal with an environment several orders of magnitude more crowded, full of human users who are 1. not communicating, and 2. not following the 'rules of the road'. Most of the drivers around here seem intent on violating as many driving regulations as possible in the most unpredictable way possible. How is any programmed system (or even a learning AI system) supposed to figure out what some muppet intent on his next text will do in his ignorant bliss?