I could give some long detailed explanation, but as it's been said above, art is subjective. It can be drawing, sculpting, music, writing- you either think something is art or you don't. But art is a sign of expression, and so long as one expresses themselves creatively, it IS a form of art- and that includes anime and manga.

Besides, if Jackson Pollock is famous because he invented the art form of simply splattering paint on a canvas, then likewise people who've become famouse through anime and manga styled art shouldn't be discredited.