Because, as my dear friend DSK has pointed out to me several times, you are expected to play at the game's pace, not your own. You'll do much better if you let the enemy come to you rather than running around like an idiot. Unfortunately, other players see that kind of tactical thinking and on-the-fly strategizing as unfair because they don't think they can do it. MW2 is intended[footnote]It fails pretty categorically at simulating small-squad tactics, but hey. Points for the effort.[/footnote] to simulate small-squad tactics, but suffers from a tendency towards lone-wolf syndrome (rewarding risky actions taken by lucky single players) while simultaneously punishing an entire team for a lone-wolf's actions when they fail (in the form of broad-range killstreak rewards like the airstrikes and especially the AC-130, not to mention the Nuke).
This is part of why, in my opinion, Modern Warfare would have benefited greatly from dedicated servers[footnote]The other part, of course, being CHRIST ON A CROSS P2P ARCHITECTURE DOES NOT WORK FOR MULTIPLAYER GAMES FUCK.[/footnote]. People who aren't interested in learning the deeper parts of the game can play on JFF (Just For Fun) servers while those like myself who enjoy the more tactical aspects of the game can play on Team Tactical servers and the like.
Anyway, if you're more interested in playing it slow and sniper-like, that's fine. Don't let other people dictate the pace at which you play the game. You'll do much better (in my experience) if you let the other team run around like chickens with their heads cut off while holding defensible positions. Build a close team of friends who can watch the other angles and talk to each other. My group uses Skype (because Steam's VoIP service is still pretty terrible). You've got to trust each other, though, or it all falls apart.
But most of all, remember it's just a game. You're supposed to be having fun and, if you're not, figure out why and fix that. Happy sniping, Geist.