I'm sure I'm missing something about the business, but I've wondered why you so frequently only get a year of paid DLC for a title. They burn a lot of resources

I'm sure I'm missing something about the business, but I've wondered why you so frequently only get a year of paid DLC for a title. They burn a lot of resources making a game, then abandon it to do it all over again. They'll repackage Skyrim a half-dozen times to sell it again, obviously people are playing it. Why aren't they pushing new DLC for these people?