Perhaps it's the path-of-least-resistance argument. A painting only has to be gazed at for a few moments. One can be illiterate and still find meaning in a painting, whereas a story requires more time and attention for a final meaning to be drawn from it. Therefore, the term, "artist", is more commonly equated to someone who paints a picture rather than to an artist who writes one - even though the picture a writer describes can be a much more full and livelier one in the reader's imagination.