Several people have already argued that review sites aren't particularly biased and that games like Mario, Zelda, and Halo really are better than Uncharted and R&C.
I happen to agree with this, but let's assume for a moment that, in some objective sense, Uncharted gets more things right than Halo does. I would argue that the review scores make sense even so. It absolutely makes sense to rate otherwise identical games differently if one is an established and popular IP while another uses unknown characters.
This is because no one reads reviews to make conclusions about games as art. It's a tiny minority of gamers that even believe that games can be art, and even fewer really care whether or not a given game is good art. People read reviews to get an idea of how much they would enjoy a game (in order to make purchasing decisions) and review scores are meant to distill all of the information in the review into an easy to understand format. Almost all of us find that average review scores correlate very, very well with how worth purchasing a game is.
So we then have to determine what makes a game enjoyable and worth purchasing. It's readily apparent that IP matters. A huge part of the appeal of Smash Bros is that it's Nintendo characters beating each other up. I wouldn't enjoy the game as much if it didn't have Mario, Zelda, and Samus in it, even if it had characters with identical move sets but different skins. Part of what makes Mario games so enjoyable is the very fact that they are Mario games, and it absolutely makes sense to factor this into a score. A big part of what made Halo 3 worth purchasing for so many people is the mere fact that it is the sequel to Halo 2. Any honest review has to consider this information.
Moving on,
Million - you've just prescribed a time-consuming process when all the average person wants to do is google the game's title, make sure it's decent, and pick it up from a store. Further, no one can do that for all games - one has to pick and choose which games to take an interest in in the first place, and at the very least average review scores provide a useful metric for this.