We humans are almost unique in the animal kingdom in our ability to recognize an object of the world in a two-dimensional image. It is remarkable enough that we can do this with a color photograph. Stretching this capacity to its greatest extreme, we are capable of seeing a simple outline as a solid form, and that just outright fascinates me.
It is so easy for us to “see” a solid form in a line drawing that we don't recognize this miracle of brain power; but on some level the brain recognizes its own achievement and celebrates it. That is why we enjoy simple outlines as much as we do. We see that preference everywhere, from animated cartoons to paintings that feature lines which are not completely necessary, as in the paintings of Botticelli, Picasso, Cezanne, Van Gogh and hundreds more.