I blame MMO's. The problem with MMO's is that they, by definition, are a never-ending world. Ideally, they are supposed to change, grow and adapt offerring new challenges and new opportunities, unlike static single player games (where there is a "finite" goal, whether it be the end of a chapter followed by a sequel or just the end period).
Unfortunately, this simply does NOT work, and it has left a sour taste in (mine at least) mouths of people. Take WoW for example. Sure they push out content now and again, and slog out an expansion every year or so... but to what end? First, I got to level 60... saw the world, did a modicum of PvP, and that's it... end game. You do the same thing over and over and over again just for a piece of armor, reputation, status, whatever. The game simply ended, but left you with a sense of needing to accomplish something because, well, there were no credits rolling at the end. Throw out an expansion, and boom - same thing... another 10 levels of "ooh, shiny!" and then the boredom starts all over again.
The problem is two-fold... developers are looking toward MMO's as cash cows that they can exploit as an addiction (social + gaming = addiction for most). That leaves less incentive for devs to be creative and tie in a riveting story and have the game revolve around it, because in an MMO, the story is made by the player with only a rough outline provided by the devs. And this mentality has fallen into single player games too, there is simply less incentive to make a compelling product, as there is just money to be had.
Couple that with every genre being so "played", and you have a recipe for bordom.