Fireflybob has probably hit the nail. Liability. There is an interesting discussion going on in the insurance world about the ramifications of driverless cars. At the moment we as humans are insured against the mistakes we (or others) may make on the road. Driverless cars will transfer that liability to the car manufacturer. I suspect airlines will happily offload the transgressions of their pilots onto Boeing and Airbus. Whether Boeing and Airbus will accept the liability probably explains the lack of pilotless planes around.