There is no true continuous commitment to singleplayer games. Their developments are mostly ad-hoc project based: once a game is out, job done and its publisher usually sits back and wait for revenue to turn, and, more often immediately move on to another project without looking back. I start to consider the bugs are not necessarily bad thing. They force developers to stay on supporting a published game. Occasionally expansions would be released, only if a game is exceptionally popular, and are never enough to really extend the longevity of a game.
I see that EA is often critized by many here, including me, for its rather "aggressive" business practice. However, I also admire its strategy of DLC development for Dragon Age, Mass Effect 2, and Sims. I am so amazed how a singleplayer game can enjoy strong community support for a rather long period.
Borderland by Gears of Box is another good example, but that doesn't seem to be how Gears of Box usually do business, at least not for the Brothers in Arms franchise.
And I am sure that developing expansions for a released game requires less input than development of a new IP / new installment for a franchise. Of course, there is always exception, like the COD, for which Activision shamefully recycling same formula over and over again and always charges for inflated price.