Who benefits from writing Amazon reviews? I've written them for years for products I purchased, for no other reason than to let other people know.
Even without that, Amazon itself (via the Vine program: 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...
The big hint is that they'll have an overwhelming number of 5 star reviews compared to the competition and often average around 4.95. Some are actually quite mediocre. So I'll usually avoid anything above a 4.8.