You not only have to consider the money spent on shipping it back to them at their own cost but also then deciding whether the product is still in good enough condition to be resold or most likely it would just go wholesale directly to a business that sells returns at a cheaper price and they handle that checking / repackaging. That by itself still probably costs money to handle. At Amazon scale, all of that together is a lot of money.
If the amount of profit they made from selling to you is offset by all those costs, why not let the customer keep it and get free goodwill and repeat business?
But that's increasingly offset by an inability to find quality products.