Some of it is down to third party sellers using Amazon for fulfilment. Amazon charge a monthly fee to hold the stock so you get to a point where your products stopped selling but you're still paying Amazon for warehouse space so it works out cheaper to dump it.
The returns stuff is weird. Surely you can re-sell that stuff. I mean they've got the platform to do it but even if they don't want to themselves surely someone would take the lot off them to sell. Failing that most of it could go to charity.
Maybe need to look at something like a disposal tax so sending stuff to landfill isn't the cheap option.
If you give it away it stops people buying the items, my company have a lot of parcels that don’t make it to the customer for various reasons they used to be put on pallets and sent for sale disposal in Germany but brexit has put an end to that as every parcel needs an invoice, have no idea what happens to it now. Amazon used to fly the books back to the USA for disposal so they might as well burn them here.
It goes on all over the place, we regularly scrap out stuff and then order the exact same thing, most of which gets scrapped out again. It's not uncommon for us to change the design of parts while still waiting for a supplier to fulfill an order, thousands of pounds of parts literally get delivered and half hour later are chucked in the bin.