Well, it is more expensive to have more than one. New fly by wire aircraft usually have 3 separate systems (sensors, computers and so on) voting to know which p

Well, it is more expensive to have more than one. New fly by wire aircraft usually have 3 separate systems (sensors, computers and so on) voting to know which part fails and continue on working, or if everything fails do it to a safe-state. The protections are gone but you can fly the plane with direct inputs. Compare this to the ancient 737 where instead of the airplane detecting errors the pilots are supposed to do it by cross checking two flight computers. Thus we see now after the crashes Boeing forced to implement cross checking in a system not designed for it because things reliant on a single sensor has ended up being classified as safety critical.