They probably have a larger development team than most, and one that works outside of the conventional X number of months, and work regularly on the game. Patches and such keep the game dynamic, aside from all of the over-the-top plethora of options.

I could throw in psychology, too; the most addictive things are those that reward you spoiradically, rather than in fixed intervals. Gambling (and probably most RPG randomizer systems) are exactly that.

They probably just go about it like normal games, in terms of what order things are done in. They just create multiple versions, and continue adding on as desired.