A lot of the time your packages are in shipping, they aren't being handled by people. They're going down mechanical conveyor belts and such. I've working in a lot of different shipping places and I'd guess that most damage is due not to human negligence but due to problems with the machines. I've seen a lot of boxes get hung up and smushed on the conveyors. Once one gets stuck, the resulting pile-on behind it will totally flatten the first box and do a lot of damage to all the other ones caught in the backup. I've witnessed little to no actual negligence on the part of the human handlers.
I think a lot of people picture their package going from one set of human hands to another, when in fact, it's carried by machine more than anything else, and machines don't know or care what's in the boxes.