Let me preface your question with another question, why do any of us play video games? Honestly, I'm asking this in all seriousness. Feel free to answer at your leisure. Why do you play video games?
You bring up the argument that in Call of Duty that you learn to aim and "kill people" (nice imagery there btw.) However, this is not the same as being on an actual battlefield, holding a real gun and aiming as you fire with it actually kicking you back. Sure, you get a virtual feeling of it if they programmed it just right, but it's still not 100% the same experience. However, you get enjoyed out of playing Call of Duty, like many other games in the world. They enjoy that rush of being on the battlefield, gun in hand, ready to take down their enemy for the cause they feel is just. It's fun, and it can also be realistic, get the blood flowing and heart rate up.
Minecraft on the other hand is a completely different game, but it's still a game. If all games were Call of Duty, hell, I wouldn't be a gamer. There are multiple genres out there for what people like. Minecraft is a genre where you build. That's all you do. You scavenge for materials, you build, whether it be a house, a proposal for your girlfriend, or an internet meme. Your imagination is practically your only limit. That's what makes Minecraft fun to me, and I don't even play the game.
Minecraft teases your imagination so deeply, you can get lost mining for the right materials to perfect what you want to build, want to do. It's not just going out and shooting people.
Gaming is an artform Arcanine. Everyone appreciates art differently. Some people might love Picasso, while others just don't understand what the hell he was smoking to come up with some of his masterpieces. Art is purely subjective. While you might love Call of Duty, others might love Minecraft, and just you asking people why they love it, won't have you fully understand why.
In my opinion, go to Minecraft's website. There's a free build of one of the earliest patches of the game you can play on your browser. You don't need to gather resources or anything, you can just build. Give that a try, and see what you think of the game yourself. You're not going to be able to form an opinion of the game without trying it yourself. It's like asking someone what they thought of the Mona Lisa or Citizen Kane without going to the Louvre or putting that DVD into your TV yourself and watching it.
So I come back to my question that I posed at the beginning, why do any of us play video games? They're fun. They tap into our imaginations and let us experience roles we could never feel in the real world, or that we could but we are afraid to try it in real life because it has very serious consequences. They teach you, they take you for a ride, they're lovely, and just plain awesome. That's why I play, and one day want to design video games. I hope this sums up in some way why people probably love Minecraft.