Also they didn't add it to their games cause as above the rating goes up but also they'd have to make different versions of the game. Some countries lile Germany and Australia don't allow games released in their country that has blood or suggestive themes. For example Hearts of Iron 4 is desensitized to remove all nazi emblems or flags. Germany also doesn't allow blood and gore for total war. I had to buy my friends dlc and gift it to him lol.
Np, i work in game industry so i know the amounts of cash it would cost them to make different versions of the game. Its easier to make a dlc and let those that want it buy it. It's just a marketing thing really.