I think you might be trying to compare Graphic Design to what we euphamistically call “graphics design”
The little “s” makes all the difference. Sigs, photomanips, etc are not Graphic Design.
Graphic Design serves a communication purpose and by definition is thought through critically.
“graphics design” is just pretty pictures. IOW, Art, not Design. While they may have a theme, they generally do not serve a purpose other than pleasing the artist and/or fans of the genre.
Don’t confuse the two. They are not the same thing.