Things are only perfect in car commercials.
Other sites give A+'s because they're actually in the game promotion business. They sell good news because that's what people want to hear, and it makes publishers happy, too. Even when they don't give games 100s, they give them 79s which they call "Great" and tell you "If you're a fan of football, you'll love this football game" like you even needed to read that.
But if you look back through all of a given site's 100s (the list probably includes GTA, Halo 2 and World of Warcraft), all of those great games have real flaws. GTA had some jacked up missions that were not fun to deal with and pretty mediocre gunplay. Halo 2 had a linear, easy single player campaign, and World of Warcraft has a terrible PvP honor system, as well as an untraceable overarching plot.
These are some of the greatest games ever made, but their problems aren't miniscule, or even subtle. So why give them perfect scores? Why, for these games, do you decide not to be a critic? And if you aren't being a critic, what are you? At best, you're free promotion, a.k.a. a fan, at worst, you're paid promotion, a.k.a. a shill.
Game Revolution will never give an A+ because we will never stop being critics.