There is no question the technology exists for an A/C to fly from A to B on it's own .. UAV's etc. The difference is the "what if" scenario .. a machine alone is expendable, a human life is not.
Public transport A/C require (and have) a minimum of duplex redundancy, and this includes the pilots. This assumes that if 2 systems are fitted then one will fail without significant safety consideration albeit with probable commercial implication. It is also assumes that if only one system is fitted to an A/C that if it fails it has negligible safety implication.
Recent technological marvels have had well publicised failings, and yet were presented as operationally ready. In the end it is my belief that passengers will not get on an A/C that has only 1 pilot .. Automatic trains/ metros are irrelevant as any failure automatically brings them to a halt until technical assistance arrives .. this luxury is obviously not available to airliners, and therefore this is not a realistic comparison.
If all else fails a human pilot has an input of self preservation that computers do not ... for example if faced with visual pending doom the human pilot will take avoiding action regardless of what the "machines" are saying ... a computer will only obey what the sensors are telling it .. and it will fly into a mountain if instructed to.