Keep in mind that there is as much stuff, of various sizes, arriving as there is departing. That distribution center might have a hundred thousand suppliers for all that stuff. Each one packs their goods differently from the next.
Receiving / replenishment is probably a more difficult job than fulfillment. With fulfillment, you need someone to put a sticker on an item and toss it onto a conveyor.
If you were to see a robot anywhere, I could envision a box making / packaging robot being deployed. But, to the best of my knowledge, Amazon packs boxes by hand. They don't do it because they are lazy or inefficient, quite the opposite.