Wouldn't a simple explanation of line drawings be that segmentation of images into shapes is an important part of vision, using whatever information is available? The reason it would be somewhat invariant across color and lighting is that those change so much. (eg. we can see by moonlight or faint illumination, when color signal is absent)

In some cases, simple segmentation fails (like with the shadowed face in the article), and you have to rely on other features.