Because you just don't play ArmA, ArmA 2 and Operation Flashpoint like CoD (just ONE example). It's an entirely different gameplay and, dare I say it, genre.
While CoD is an arcade/arcade-ish - shooter which focuses a lot more on Player vs. Player.
ArmA, ArmA 2, OFP are the closest what you get in terms of war-simulators on PC. Like the name suggests, Bohemia Interactive Studio (the dev studio) want to simulate real war, it's hard to integrate Team Deathmath, CTF or Assault in a simulator game, although not impossible.
If I recall correct, Bohemia Interactive Studio said in some interview way back then, their focus is more along wanting players to play together rather than against.
Being a war simulation game is also its weakness because of that it's not so wide spread like CoD, Counter Strike or Battlefied, it's "too complicated" to play if you will.