In an unregulated market, brand identity and reputation becomes a very strong differentiating factor. The strongest brand on Amazon is Amazon, so by allowing the generic market to rot they are allowing their own-brand basics products to establish themselves because consumers can assume that Amazon products won't have this fake review problem.

It does also work for other brands too of course. I was in the market for some Lightning cables recently and almost every single cheap cable was a switch-out with review for different products. The only brands I recognised were Amazon and Anker and both their stuff was legit (I bought from both).