Talking from an engineering perspective, planes rely on sensory input to determine their state, say their flight path, and what actions to take to ensure they are on course. In the case where sensors pick up false readings, we have an intuitive source of information (i.e. the pilot) to correct the plane making mistakes based on bad input.

Say what you will about drones and driverless vehicles, but the catastrophic impact a out-of-control plane would have is too high no matter how small the risk is. All tech is fallible - we just test as much as we can until we think it can handle all of what we know it is supposed to do.