Because adding a battery backup means one (or two!) more batteries that have to be tested and certified to fly; more batteries that have to be checked before fl

Because adding a battery backup means one (or two!) more batteries that have to be tested and certified to fly; more batteries that have to be checked before flight; more points of failure that could ground an aircraft or cause incidents themselves; more weight on board (especially relevant for smaller craft); etc. etc. The battery issue isn’t idle speculation; a recent flight I was scheduled for was cancelled when the airline accidentally allowed a battery to deep discharge, necessitating a new battery to be flown in from another city (delaying the flight so badly it was cancelled instead and passengers delayed to the next day).