Oftentimes this is done to circumvent paying patent license fees- for example if a patented component in a BOM would cost 75 cents per unit from the manufacturer, and the in-house team found a way to perform the equivalent function for 15 cents then it would instantly allow your product to undercut the competition. In Amazon's case, all it takes is a query to find high margin items in which knockoffs can be made and self-promoted to eventually outrank sales of the original item.