It's interesting that you mention fan-fiction. I imagine the reason people overlook forking" fiction stems from the fact you're consuming the story, remixing it and spitting out something new. On the other hand, forking code is more like tacking on a paragraph here or there.

Also, while you did touch on the hacker culture I think that's the most important factor. As hackers we're practically born to tinker. When we tinker with hardware ,say a remote controlled car, no one confronts us and asks "Hey, man, why are you tinkering with that car?" More often than not you'll get people saying "Hey, that's really cool!"

Furthmore, it's easier to take a get tangible changes from tinkering with a program than with a piece of literature. You can't miss the impact of gravity on gameplay in a game like HATETRIS but very few people will pick up on the significance of a new piece of text in the middle of a novel.

I really like how jsankey put it when he said "If you put something out there, you should be flattered that people take enough interest to tinker with it."