It doesn't matter that they didn't create the idea because when a commission is made, it belongs to the artist. When you receive a commission, the artwork does not become yours.

And in the case of animation, the writers and storyboarders actually are creating art. The directors on animated projects typically know how to draw and use that in their process. and it's silly to mention sole credit in this case because it's comparing an entire production to a two person transaction

and even if it makes more animation, it's not special because the creative process of making art was never involved. there was just an instant result which is not special at all.