I assume it's not done because the tuning is affected by the pressure. Bagpipes typically play melodies over constantly sounding drones, and different sized pip

I assume it's not done because the tuning is affected by the pressure. Bagpipes typically play melodies over constantly sounding drones, and different sized pipes are affected differently by changes in air pressure, so bagpipes will sound out of tune if the air pressure varies too much (tuning errors are much more obvious in harmony than in monophonic melodies).