It depends a bit on your safety standards. There are already autonomous flying things delivering blood and blowing up oil depots where it doesn't matter so much if stuff goes wrong, but to be an airline pilot you have to know how to deal with a huge range of emergencies and systems packing up.

With a car if the engine fails you just pull over. With an airliner it's not so simple. As a result the training for a pilot is much longer than for a bus driver say.