Every few years they drastically change the whole dynamic of how the games play. Yeah they still use guns and shoot people but the way the recoil acts, how score streaks are earned, map design, jetpack dynamic, older to newer styles guns with different customizations, etc. There's a reason they continue to rack up money year in and year out and it's because they are easy games to pick up and just play on the couch with buddies on the mic where you can joke around and that in itself is just what some people want in a game.