I was not aware that Metacritic was such a big issue until now, but I think I have a pretty good idea why people put so much weight on the numbers for a game review, more so than any other genre.
Unlike film and music, there isn't a cheap way to enjoy a game outside of out and out piracy. There's no dollar theatre for video games, people aren't playing games on a radio, and video game rental is just not as simple as it once was. Games are priced so high that players are demanding a certain level of "excellence" out of them to justify the $60 purchase price. I can't imagine impulse buying a game over $20. Impulse buying a $20 DVD, well they're all about $20 now, aren't they? If the developers would look at their model, spend less time worrying about pushing the technological envelope now that that's pretty much over and done with, and use that cost cutting measure to release a series of cheaper games, I think people will have fewer qualms with playing a mediocre game. Tons of people play Angry Birds, and what is that? Bare bones gameplay, and a price point to match it.