The problem is that they have an obvious incentive to err on the side of positive reviews. Because if every product on the site has a 1 star review few people will buy anything. But if most of them have 5 stars people will be much more eager to purchase.
Not only that, but if you get people buying lemons from scammers, some percentage will forgo the refund process and re-engage with Amazon to buy something else: Amazon gets a cut of that too, plus more eyes on other products during that process. And even if there is refund, the platform will get that money back from the seller anyway.
