I know I myself do not personally hack; I am never accused of hacking. I don't understand why anyone would hack - the entire point of playing the game is to play a game and if you are hacking you aren't really playing a game but instead just wasting time.
With that said, some of us are very, very good at this. I've been playing this version of COD since it first came out, and while I have other versions of COD this is by far my favorite. I have been a clan member with three different clans for years now; I play EVERY day.
First person shooter games all have the same strategies which are exactly the same in real life combat - Know where you enemy is at all times and make sure they do not know where you are. The perks you pick are important, and what you need changes during the course of a single game. Late last night I came onto the server, for example, and there was only four other people on the server so dead silence and a silencer was important (What I call my "ghost" class). As more and more people joined the server sound mattered less, UAV blocking mattered more, and a class with more firepower was required. Most people do not even consider these things.
I was a combat instructor in the Marines; I taught people tactics. I practice "emergency action drills" so when "play x on Crossfire does one action" I automatically have a reaction. Also, I practice random things - recently I've noticed I'm tossing smoke nades over buildings and then run around to see exactly where they land. I've also been playing with classes that have more fire power so I can nail people through walls (I call this the "McCheese Beast Mode" class, which was taught to me by one of our clan members).
The tactics really kill me on the server. Sometimes you can kill someone without having seen them. You see shots coming out of a building, you know exactly where they are standing because you've fired from that spot a thousand times, and shoot them through the wall. It's not that you are hacking, but instead because you know exactly where they are standing from the stream of fire coming out of a window.
Also, ping helps.