It’s true you will never get rid of all of it but, just like crime, basic enforcement is a deterrence. They know who’s buying, they know where they’re shipped, they have a fair idea if they’re returned. Just requiring reviews to be from purchasers after they’ve received the product, removing positive reviews for returns without replacement (or flagging them as returned), and a few other steps would make fake reviews either very expensive or very expensive for the results.

The fact is, Amazon makes most of their money on AWS, and I don’t think they care to put in the real effort to make their marketplace trustworthy again. Without that, it will continue its downward spiral.