I'd be very surprised if ANY MMORPG didn't constantly iterate as you describe (all the current major ones do, at least), and virtually all multiplayer games all

I'd be very surprised if ANY MMORPG didn't constantly iterate as you describe (all the current major ones do, at least), and virtually all multiplayer games allow for balancing at the very least. The problem is that most single-player games are finite. Once people have completed them, they're done. Replayability is something comparatively few people can stand for any length of time, especially in a crowded market. Add the problem of an exceptionally short tail for most games (20-50% of total sales in the first month would not be atypical) and the value of updating the mechanics of your single-player game becomes much less valuable. Best to make another game in the series that improves all those flaws. A few exceptions, all indie games: Minecraft, Eufloria and Darwinia. All of these have had major updates to gameplay over time.