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) 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...

Those are clearly labeled. And Amazon Vine reviewers for the most part do make an honest effort to review the product. I tend to ig ore them in favour of those 3-4 line reviews in small caps and iffy grammar. That's a much better signal for reliability than a blog post length Vine review.

By labelling some reviews as paid reviews, readers are fooled to believe other reviews are not paid.

Some offer discounts and gifts and such for a review. They don't even say you have to give them a good review, and so they'll justify it as an anti-bias thing. But you have to show them the review, and it's a little odd to give someone a bad review right after they offer a free gift.

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.