I've noticed the same thing. Part of it might be that reviewers are reluctant to alienate fans of [thing being reviewed]. Another explanation is that they are intuitively norming against a wider degree of things than they actually review. For example, I was buying a smartphone recently, and a lot of lower-end devices I was considering had few reviews, but famous high-end brands (like iPhone Galaxy S, etc.) are reviewed by pretty much everyone.

Playing devil's advocate, it might be that there are more perceivable degrees of badness/more ways to fail than there are of goodness, so we need a wider range of numbers to describe and fairly rank the failures.