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.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
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