I tell stories the way I want to. My narration is an integral part of the experience. Being able to write a story as if you really are an observer telling somebody, for me, is part of the fun. There's a certain magic in putting comments and personality in narration, and it's part of my fun and presumably most of the readers' fun as well. Writing for a visual medium would kill that for me. I could do it, but my usual, preferred storytelling method would clash violently with it. Especially because narration, imo, never, never, works when you could just show someone. That's the point of a visual medium. I don't really care how hard it might be, that's just not my bag, for the most part, though I have written screenplays before.
Compounded by the fact that whenever I get someone to make a visual thing for my stuff, they never get it right and I just end up using it as a rough approximation.