To give them a bit of credit, Amazon did ban giving reviewers cheap or free products in exchange for reviews a few years ago. They also shut down 3rd party sites that were facilitating the buying and selling of reviews.

https://www.consumerreports.org/consumerist/amazon-bans-most...

https://www.bbc.com/news/technology-32251698

I bet there is more they could be doing, but you can probably never totally solve the problem.