The DLC isn't completed, hence why it isn't in the game. Not everyone at a studio works on the same part of a game at the same time. So while the game goes into the testing stage, other people can start work on DLC to be released later down the line. That's why DLC has staggered release dates, it's all constantly being worked on in one war or another. Hence the DLC coming out in march and beyond, rather than right after release.
Now, if the content is on the disc and you simply need to pay to unlock it, then yeah, that's a big load of bull.