Because most of the DLCs is just a way to milk hardcore fans. It is actually very rare when a DLC adds something valuable to the main game, expends the story, etc. Most of the time it is either:
1) Something that should have been in the game at the time of release
2) Useless "cosmetics" or "additional multiplayer maps"

And when you start charging the same amount for DLCs that you already do for a game itself this is just plain robbery.