Because it's an easy, thoughtless comparison that's why.
I always laugh a bit at the COD4 comparisons.
The game is being called Killzone 2, but it's the spiritual sequel to Killzone Liberation, which featured boss fights, a variety of enemy classes, and impressive enemy AI--stuff the COD games ,1 through 4, have never had.
The boss fights are what I want to see the most;given the audio-visual standards of this game, GG have the potential to create some balls to the wall, high quality boss fights.
In fact Guerilla has hinted that they striving to do just that. They claim the mini-boss ,the Heavy, is just a small taste of some "much larger" boss fights.
I wouldnt be surprised if we saw some MGS inspired encounters in KZ2; Liberation has a mech influence reminiscent of Metal Gear and the developers say that they are big fans of that series.Anyone's that's played Liberation shouldnt expect Killzone 2 to play out like COD4, because that would mean GG would almost totally abandon the design philosophy that made Liberation such a great little game.