Even WoW goes through these phases, where everyone burns through the content they enjoy, and then leaves/hibernates until the next patch. I don't like it, but its the nature of these kinds of games (WoW style MMOs), which revolve around a linear consumeable content ladder. You "beat the game" and then you feel done.
People saying stuff like "Have you done every FATE and leveled every craft class!?" don't seem to comprehend that people only want to do what they enjoy, and when they run out of that, there's nothing left to keep them in the game. Its like saying "Keep playing FF10 to see how long you can run around in circles without your thumb getting tired! That's infinite content right there!"