I used to work in a shipping depot that delivered to Amazon, among others.

And whenever a shipment went out to Amazon, we had to take packages of 20 items, like they’d be delivered to retailers, and rip them open to put each item into packaging carton individually, before it was even delivered into an Amazon warehouse.

In our case, it was parasols, so it could be different for smaller items or just items that are more likely to be ordered in bulk, but yeah, I imagine, Amazon just does not want to deal with the packaging in their warehouse, if possible.