Realism is overrated. They're videogames for christs sake.
Arcade shooters are overrated. They're a dime a dozen.
Clue is in the title: They're billing the games as 'modern warfare' when what they're actually producing is a game that allows people to dual wield shotguns, submachineguns and survive hails of bullets, with cover being an optional extra. The fact that the story has been used multiple times before, most noticibly and recently by WiC just shows that they're lazy. Any pretence to 'modern warfare' went out the door as soon as the CGed russian boots hit rendered American soil. At least stuff like Red Dawn and WiC was set during the cold war when this was a plausible (if unlikely) scenario.
It's not like the world doesn't have dozens of actual conflict hotspots that could be utilised.
Obviously MoH MW isn't out yet, but I expect it to continue in the vein of the previous games where a ridiculously hard one man army kicks wave after wave of enemy ass.
Notice how I didn't bash Halo for being unrealistic, I only bashed it for being shit.