Why don't readers do reviews?

I can't find an answer to this one. Even when a review is requested in the back matter or readers are getting a free book.

I rely on reviews to give me a sense of whether I'll enjoy a book. I've also discovered some great books from reading friend's reviews on GR.

So why don't people do reviews? In this world of constant sharing and social media, why not share your reaction to a book? Even a simple "it was good" is great. I don't get it!

So why don't readers do reviews?