If FarmVille isn’t all that great of a game, why do people play it? It’s the same reason people go on vacations they’re not really looking forward to. The "sunk cost fallacy" is a strong, almost primordial thing, and it helps to understand it.
You Are Not So Smart, the go-to blog for understanding why we all do silly things, tackles the psychological roots of sunk cost. Put simply, it’s the feeling of "I’ve already spent X, so I’d better spend Y to salvage it." David McRaney’s post makes good sense of how FarmVille is a master manipulator of the sunk cost emotion.