|(Image source) - cause I can't draw!|
Because they know what should have happened.
Thirteen years ago Douglas Adams wrote a prescient essay called How To Stop Worrying And Learn To Love The Internet (still available here), about how the internet was not changing culture so much as returning it to pre-consumer times:
The internet lets us as the audience participate in a more direct way than we've been able to since the advent of recorded entertainment. Often it's a peanut gallery of forum comments, but sometimes someone in the audience feels so strongly about something that they have to make their own mark on it. They write fanfic.
This subjective view plays odd tricks on us, of course. For instance, ‘interactivity’ is one of those neologisms that [BBC journalist] Mr Humphrys likes to dangle between a pair of verbal tweezers, but the reason we suddenly need such a word is that during this century we have for the first time been dominated by non-interactive forms of entertainment: cinema, radio, recorded music and television. Before they came along all entertainment was interactive: theatre, music, sport – the performers and audience were there together, and even a respectfully silent audience exerted a powerful shaping presence on the unfolding of whatever drama they were there for. We didn’t need a special word for interactivity in the same way that we don’t (yet) need a special word for people with only one head.
I've been watching the Young Justice fandom from a distance thanks to a friend of mine, and it's been fascinating watching week to week as they spin off little realities trying to extrapolate what will happen in the future. Every Saturday at 10:30 AM EST the lid is removed from Schrödinger's Episode and the status quo is reasserted only to spawn another hundred realities that Should Have Been. When those potential realities seem better than the one that comes to pass, fanfiction is born. I've done it myself, most notably in the Transformers: Armada days when my own ambitious ideas about the Mini-Cons were unmatched by the official fiction. (Scoff if you want at my delusions of grandeur, but the official fiction was mostly terrible.) Sometimes as a fan you're caught up in a feeling of how things should be that has to be expressed.
The internet has brought us back to the normal human state of audience interaction in art, and fanfiction is one of our best tools for commenting on how a story should have happened, if only the original writer was without editorial constraints, had more vision, or understood just how much Breakdown and Knockout were OTP.