As a mod for 2 defaults (on another account): It's hard to describe the psychology of default-modding and why one keeps doing it despite it basically a complete time suck for no pay. The "power" people talk about is so trivial I can't seriously believe it's a real draw for anybody who has been modding for more than a week (I mean, so I can delete comments, whoop de doo, so what?) Modding very rapidly becomes just a boring daily chore. But there's a number of factors that keep a mod going nonetheless (for me at least):
you helped build a community and a credibility and a platform that you don't want to abandon;
you sometimes develop a very narrow focus on just your sub. My modding account is subscribed only to the two subs I mod and their two associated mods-only subs. When I'm on that account I literally see nothing else of reddit. I will bet the /r/science mods really are thinking only about the logistics of the science AMAs, not reddit as a whole. (Admin problems? Reddit's been taken over by Satan? Whatever, we have to keep our AMAs going.)
in some subs, there is genuine idealism about the educational value of the sub, which can even develop into a belief that somehow humanity will suffer if that sub goes dark or isn't modded properly. (People will be deprived of science news! CIVILIZATION COULD COLLAPSE!)
sometimes modding's just become a habit (for a long time it was just the thing I always did to kill time during my subway commute)
a big one: you bond with your fellow mods and don't want to abandon them. You're chatting constantly with them, in the separate sub that only the mods are subscribed to, over modmail, in PMs and often in IRC chats. It starts to feel like a little band of buddies really working together. Anyway, point is, each mod that quits means the workload gets higher for the other mods. The only reason I still mod at all is the periodic waves of guilt about how hard I know the other mods are working. Fortunately for me (but not for them), those waves of guilt are occurring less and less.