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.
Another approach ... they could also identify which products either have wide-supplier diversity for the same thing (commodities) or narrow supplier density with many branded variants (OEM suppliers). In either case, they can go direct to the manufacturer without ANY innovation, slap the label on, and cut out the middle-man/sub-retailer costs. I think Amazon, in particular, has a team analyzing these factors as input into their sourcing (on top of general considerations like margin).
