For a ton of reasons. Fake or purchased reviews is a big part, but also the fact that honest reviews are often misleading or incorrect. As one example, I've often read reviews where the people submitted them before or immediately after they've received the product, and so can't possibly have enough information to write a useful review. Or reviews that are often skewed by price point, encouraging people to rave about crappy products simply because they were cheap.

Amazon reviews are a great example of all of these problems, I think.

If the reviews are about an employer, then there's a whole other level of problems with them. Perhaps the reviewer didn't get along with their manager and so have an honestly terrible opinion of the place, but really, the place is generally great. Or perhaps the opposite -- the reviewer got along wonderfully with their manager and so they're ignoring all sorts of real issues with that employer.

> What if there was a cost attached to posting a review?

I don't think that would help. It might even make things worse, because it would filter out more ordinary people and tilt the balance more towards paid reviewers.

I mean, I could be wrong here. But I've not yet seen a place that allows the general public to submit reviews where the reviews were helpful or accurate in terms of figuring out if the product or service is worth purchasing.