Huh. I guess, now that I'm thinking about it, "nerdy" hobbies always provided me with the mental stimulation that other stuff couldn't. I've always loved literature, and gaming (especially anything story or character-driven), as well as drawing/art. I remember when I was a little girl, I used to read nearly all the time, and if I wasn't doing that, then I was playing the original Marios or Sonics on my parents' old SNES and Megadrive, or Pokemon Red on that brick of a Gameboy. I sketched and painted a lot of random crap, too.
Nothing much has changed, but my interests branched out into history and psychology -- and don't forget utter over-analysis! -- as I grew older, so there are more "mental"-type interests. Sports, or anything "mind fluff" (not that sports is) has never really interested me. I'm not an outdoorsy or athletic person anyway, and I don't like just plonking my ass down and staring blankly at a TV screen for three hours. I just find it boring, because there's nothing to really think about. Which I mean in the least elitist way possible, by the way, if that was misconstrued as arrogant or anything.
So...I guess that's why I'm a nerd? I like nerdy hobbies because they're more fun and clevererer? B-)