Personally, I think it's just bad journalism really. I mean granted everyone's taste differs and that's what make life so interesting. And yes, some games are truly epic and deserve high marks, but when reviewers/journo's start giving everything high marks, it is either bad journalism or something shady going on... I wouldn't put it past developers/publishers etc to be involved in giving handouts for favorable reviews in order to generate more sales.
I refuse to believe though that every game coming out these days is deservant of the high scores, even the titles expected to be so. Just because it is a film written by say George Lucas, directed by Steven Speilberg and has A-list Hollywood actors etc etc, doesn't automatically make it great. This has happened in many movies before and also in gaming, but people get swept up in the emotion etc and gush.
I also think a game when just released should be reviewed and then reviewed a month later so the reviewer has calmed down and can get a realistic look at the game. To notice all the small irritations that were overlooked in the hype and excitement in the new game.
I also think the rating systems of games should be reviewed. Yes graphics, sound, playability, controls are all important, but what about things like long loading times, or patches 23 seconds after release or glitchy menu systems etc? I think too many reviewers skip over these things in the haste to be the first out with a review.