I've never understood why they make planes with 1-2 sensors for a crucial reading like airspeed. Why not have 20 airspeed sensors of 5 different types? It's an

I've never understood why they make planes with 1-2 sensors for a crucial reading like airspeed. Why not have 20 airspeed sensors of 5 different types? It's an obvious failure mode that your one sensor will fail and then the pilots and the computer will be left in a state of dangerous uncertainty about the situation.