I think there are two important factors when it comes to explaining the expecting 5 stars thing.
First, reviews are pretty rare these days. Some of the oder products on the marketplace have 500+ reviews. Even the most popular products are lucky to hit 100.
Second, people generally use the review system if they love something, or HATE IT!!!!! Very few people will leave a review if they are merely satisfied.
Together, I think that leads some to obsess over their ratings, and fair and ballanced or not, a string of "poor" reviews will have a real impact on sales.