The first reason is psychology; when I go online to write a review it’s usually because I’ve had a bad experience rather than a good one. So it stands to reason that most legitimate online reviews are going to be bad unless there was some motive for giving a good review.
The second reason is that fake reviews can now be purchased by businesses or companies by the hundreds. These social media companies will go around posting hundreds of 5 star reviews for your business completely destroying the legitimacy of an online review in the first place. They usually stick out like a sore thumb though; they’ll often use the same talking points and follow some kind of script. If you follow the profiles on these fake reviews they’ll sometimes be interconnected with each other as well.
And the final reason online reviews are almost completely broken is that you have these websites dedicated to reviews that will try to extort money out of business owners to hide the bad reviews and if they refuse they’ll flag all of the good reviews as “not recommended” and hide them in a fold out menu at the bottom of the page. I’m sure we all know what site I’m talking about, but I won’t mention for fear of getting sued.
In conclusion I’d say only trust online reviews if you can verify the identity of the reviewer, and that the people reviewing didn’t have some kind of ulterior motive for giving the review.