1. The number of people who read reviews are a small, small, fraction of total buyers. Most people look at review count and the star rating (not the number, the image).
But more importantly,
2. Ranking in search results is the #1 factor in driving sales. Everything else is a distant #3. Search engines will reduce rankings of products with negative reviews.
(This my anecdotal experience after 2+ years of being an Amazon 3P seller)
As those machines look for signals of "authenticity," review factories learn to manufacture reviews that fake those signals. If adding a small mix of two-star reviews helps with search rankings, the review factories will start doing it in a heartbeat.
describing tradeoffs that would seem reasonable to most people
(Transcript...)
Cueball: Spammers are breaking traditional Captchas with AI, so I've built a new system. It asks users to rate a slate of comments as "Constructive" or "Not constructive."
Cueball: Then it has them reply with comments of their own, which are later rated by other users.
Megan: But what will you do when spammers train their bots to make automated constructive and helpful comments?
Cueball: Mission. Fucking. Accomplished.
(from https://www.explainxkcd.com/wiki/index.php/810:_Constructive)
Unfortunately, the endgame for bot spam indistinguishable from humans isn't some benign silicon intelligence, but an epistemic war for attention, spending, manipulation, and/or fraud, benefitting deep pockets and highly motivated parties.
It ends poorly.