Niggles: I keep wondering why indie devs keep throwing out multiplayer games....
IAmSinistar: Easier than writing proper AI would be my guess. Either that or they think they're going to get a slice of the MMO-addiction pie. Kind of need a good game first for that, though. :)
3rd reason: a friend is doing some courses right now to be qualified in areas he already knows (well done UK jobs market! nobbers!)... anyways, as a part of one course locally, they actually have to design something along the lines of either a social network or a social "game" - some people opting fot the latter have gone for farmville type shit, others for MMO type shit.
I would guess that explains an amount of the glut at least if courses in other locales are also doing the same. Not all of it of course... most of it is the programmers having no ideas would be my guess.