Why is Call Of Duty so popular?

I do not understand why Call Of Duty is such a good game, its just like halo, unrealistic and lame. You could bring a RPG into a small building and have no conseqnences. For the past like 5 or 6 games the shooting has been the same, yueah sure, they added new enviroments and whatever but the shooting on almost ever wweapon is the zsame its sooooo boring and lame. Any Ways I could rant on forever about why COD and Halo suck but I want your personal opinion!