This is because games are getting more expensive, time consuming and risky. If you only sell 100.000 copies these days, then your game is a failure (commercially). It's because we gamers have kept demanding more and more stuff, for the same price - that they're doing this DLC business. In Denmark, films in cinemas have gotten more and more expensive over time, but game prices have pretty much stayed on the same level for as along as I can remember.
I don't like this DLC business, I agree that they are effectively cutting out pieces of a game and selling them separately. But I understand it and therefore can't blame the companies for it.