Is flying a plane vastly more difficult than medical diagnosis?
It is. Reaction time is essential, timing is crucial. In decission making without time contraint it is hard to beat a computer. In reacting to a surprising situation never thought of, it is hard to beat humans.
However, computers in airplanes are still very, very basic using very simple algorithms and laws. There is no doubt that a computer could do many jobs much more accurately than a human. However, the more complex you make it, the more bugs can remain hidden until they are discovered by accident (by an accident or after an accident more probably...). It is also much easier to install true redudancy with humans (they are all different, except for one in the brian movie ;-) than with computers, where preventing commom mode failures is really rocket science.