Because there are people like me who play the story and enjoy it. Have you ever looked around you in the midst of the combat and seen the little things?
This mainly applies to the flashbacks in Black Ops and WaW, but have you noticed the prisoners that you find every so often? This, out of every moral choice created, is my favourite. It has little to no effect on the outcome of the story so you have no reason to change it on your next playthrough to see what it does. All it requires you to do is think.
Do you shoot them, or leave them? It's almost a reflex, in the rush of the battle you can't tell the difference between those shooting and those surrendering, maybe you accidentally shoot those cowering in fear; you feel remorse. It's a beautiful choice that shows a lot about you as a person. I love Treyarch for this.
The stories are also pretty decent, most people just see them as shoot this, kill that, but they're actually pretty good. I couldn't put Black Ops down when I started to know more about the numbers. Modern Warfare 2 wasn't as good but it still had a good story. I wanted to know if they got Makarov, whether the Americans and Russians made friends again and I was genuinely devastated when Ghost and I were killed by Shepard. I wanted to murder him for killing ghost.
See, it's the little things that make them great. They may not be the next best story but they have their moments that drag you in and whisper things to you. They do bring a great deal of immersion to the table, in between deaths of course.