Tends to be because most games aren't for everyone and everybody rates game extremely subjectively, so you get a whole bunch of 12 year old kids who don't "get" strategy games rating them waaay down because "4X super space exploration and micromanagement" it's not as good as Call of Duty Blackops or it's "too confusing". Then you get a whole brand/game loyalty thing, where a bunch of people who rate play CoD rating BF:3 as a 1/10, and vise versa because "BF sux go play a reel game like cod."
So yeah, it ends up being pretty useless on a whole really, and doesn't give you any indication of how good the actual game is. Even when you read game reviews, by professional game reviewers, more often than not they suffer from extreme bias, to the point where I personally trust the marketing spiels to give me a better and more objective opinion. At least I know what I'm getting is a sale pitch and I can take it with a grain of salt.
It would be nice if people could review game for what they are intended to do, what they set out to accomplish and how well they met those goals, rather than pitting them in an open field and making blaze comparisons. Unfortunately it doesn't seem to be the case though