Because there are absolutely no consequences, and general boosterism increases sales. If you go to site X and there are a mix of good and bad reviews for a product, but then go to Amazon and see all good reviews a similar product, you buy at Amazon. Especially if you have Prime, which you might just have because you like a tv show.
If a systematic fraud is exposed, accounts are banned, Amazon press-releases a "crackdown", new accounts are quietly created by new business entities owned by the same people, process continues. What's the cost?