Even alone making grippers that reliably can pick up arbitrary objects are surprisingly hard (I believe Amazon did have some prizes offered to anyone who comes up with improvements there). So for now it's cheaper overall to have humans do the tricky part of actually picking stuff up and putting it in boxes, and where possible move other things to/from them. E.g. various "self-driving" shelving systems etc.

And Amazon ships all kinds of stuff in various packaing formats etc, so is kind of the worst case here. I could see a company that produces and packages its own stuff choose differently here, and design packaging specifically with its logistics robot capabilities in mind.