The reviews on this site are mostly disgruntled employees who are often fired for good reasons and fake reviews posted by companies trying to boost their rating. Glassdoor makes it difficult enough to post a review that the only people who will do so are highly motivated -- either because they were incentivized by their company to post a good review or because they want to retaliate against their company for firing them. A normal person wouldn't have the time to jump through the hoops.
They offer paid services to combat company's negative reviews. It's basically an extortion racket.