Well, it's not that it doesn't exist at all. I think of the some Brian/Justin doodles and comics and I've also seen some artwork of Gale or Randy. But it's true that it's rare. Probably it's also a matter of age. Children and young people tend to draw more than adults, and I suppose that the Harry Potter fandom is younger in average. Another reason would be the genre, because fantastic literature always leaves something up to the imagination, it's probably more "graphic" in itself (or makes us imagines pictures) where QAF is much more about the inner feelings and the relationships, so there's more need for words. However, I write because I can't draw. And there certainly is inspiration for artwork in QAF as well.