Fuck it. They sell because they're fun, both with and without friends. The mechanics of the leveling system are designed to add replayability, and it's designed so that even if you're shit there's still enough flashing point totals and level ups to stay interesting. And by the time it wears thin, a new one is on the horizon.
No one is forcing cod fans to buy these, the older ones are still popular. Obviously, if it wasn't worth it to them, the series wouldn't do so well. You don't like it. Congratulations, that means absolutely nothing. I get that you don't like cod games, so why do you keep having to prove it?
Also, lol? What are you, twelve? In all caps, no less. And the assertion that I'm trying to justify my purchase is cute too. Contrary to what you may believe, people aren't stupid, they aren't "fooled" every year by the marketing. Marketing might bring in new fans and inform old ones of the new game, but it isn't the sole reason for sales.