They aren't paying the same people. If I sell my own brand, and they make their own brand and pay someone else to make it, they're competing with me on a platform they control and they give themselves advantages - such as showing up ahead of me in search (Google got fined a lot of money for this exact practice), putting links to their products on my product page saying "buy our brands instead" (yes, they really do this, see https://www.cnbc.com/2018/10/02/amazon-is-testing-a-new-feat...), giving themselves exemptions from policies they apply to third party sellers like review manipulation bans (see Vine program), etc
