There are many companies that will pay people to write positive reviews on behalf of them or some third party (even though it's against the terms of service).

There are many companies that will pay people to write positive reviews on behalf of them or some third party (even though it's against the terms of service). Even without that, Amazon itself (via the Vine program: https://en.wikipedia.org/wiki/Amazon_Vine https://en.wikipedia.org/wiki/Amazon_Vine) will pay you in goods for writing reviews. I was a part of that program for a few years, and found my review quantity increasing and quality decreasing as I wrote more just to get more free crap from Amazon. Many other marketplaces will also incentivize reviews in various ways. You personally might have more integrity, but the internet as a whole doesn't...