Because I like telling people stories, and making stuff up. Because I used to tell my stories by running around a playground saying, "over there is the castle! Once we get to the castle we'll be safe from the bad guys! Oh no, they're gaining on us -- summon the winged horses!" but then everyone else grew up and quit wanting to play with me.

(Then I discovered RPGs. "Why do you write instead of playing RPGs?" is kind of its own question, and I guess it boils down to, "Writing feels more worthwhile when I'm done doing it.")