An important part of a product is its standard, fungible nature. Every unit off the production line the same. Quality maintained. Sealed in packaging.
As soon as you take it out of the packaging it becomes a unique item with a history. It's outside the QA bubble. It might be in perfectly saleable condition; it might have something wrong with it (after all, there's a reason it's being returned); it might have been damaged by the customer. It's a liability. If they were to take it back and re-sell it to someone, and get sued, that would wipe out more value than throwing away hundreds or even thousands of the same good.
This is also why secondhand goods in general are so cheap, and things like furniture may even have negative net value because they cost more to move than they are worth.