Why is concept art only created using raster graphics?

[MedionVuVySoft]

The title says it all. Why is concept art only created using raster graphics? Like, I have never really seen one creating concept art in Illustrator (i.e. using vector graphics) rather than Photoshop.

[Nancy OShea]

Concept art is not necessarily digital.

Tim Burton

"Concepts for an Alien Project," 1984
Mixed media

"Alien Fighting Men," 1981-1983 ——————————-

Pen and ink, colored pencil