When using a sword with Looting enchantment level 255, shulkers are only dropping 1 shulker shell per kill, as opposed to animals like pigs and cows, that will drop multiple stacks of food. This does not only apply to non-hostile mobs however, mobs like creepers and skeletons also drop multiple stacks of their respective items. Therefore I do not understand why this does not apply to shulkers and their drops. It doesn't seem like its something blocking ''rare items'' from dropping more, as Pillagers will drop multiple stacks of emeralds with this enchantment.