The people boxing the orders are moving at quite a clip, and the fewer the box options, the faster they can move. Not to mention that when they order boxes, it's A LOT cheaper to buy a million of 5 different sizes of box than 100,000 of 50 kinds of boxes and bubble mailers. Then you have to think about space, as the packagers are in these small areas and can't walk around, looking for best-fit boxes. This isn't to advocate for this approach, because it bugs the heck out of me that they're wasting cardboard just to send a set of nail clippers or a set of tweezers. It's all about maximizing speed, which means keep the workers in this tiny box area, not giving them options so they're trying this box, then that box, then this mailer, and then ordering in huge volume to get discounts on the boxes themselves.
I remember in 1990 or so, a friend of mine went to a job fair at the local community college. This guy was tired of working for other people and wanted to be an entrepreneur. So he asked various employers around the area what they needed. I remember him telling me that several said they needed cardboard boxes. They would love to have a local supplier. They had to order from across the country, and it was a problem. Packaging. It's crazy to think have important it is. It's like fasteners. Taken for granted, but practically everything has some sort of fastener involved in it.