Because it's been so in the past. The side I think people should see is that DLC 9 times out of 10 is very insignificant to the actual story / main portions of the game. The majority of the time, you can skip DLC and not even tell the difference.
DLC doesn't necessarily have to be significant or even relevant to the story. That kind of DLC is the kind of DLC people should be more annoyed about since it's basically the realization of the "Charging us for unfinished content" point people like to make up.
If Konami were to drop a chapter 3 DLC for $14.99 for MGSV and it supposedly ties up the loose ends and offers some new content that didn't make it to the original game, some people would say "Man they could've just gave us that for free!"
DLC like GTA's Ballad of Gay Tony is totally unrelated to GTA IV and yet at the same time, someone would have a hard time proving that it was just "held" back.
If a game is great and the DLC is great, why not buy it?? Games have to be finished and pressed for release most of the time so it's not always possible to fit in alternate content or w/e into a game before launch OR to simply provide it for free as that means people would have to work for free after a game's out.