Are they not Art Department? They are Artists, they are often refered by the Art Department, however the position of Storyboard Artist is unique.
In most cases, they aren't designing or contributing to the esthetic look of the film, they are mapping out the shots for the camera.
The Storyboard Artist is not hired by the Production Designer or Art Director, but by the Director or Producer, and works almost exclusively alone and with the Director. The Storyboard Artist works according to the Director's schedule, and they often work from their own studios.
You seldom find them in the Art Department, yet because of their artistic skills and connections they are part of the artistic community. Often they are given a table and chair in the hall. Sometimes their own or the Director's office. Its also common for a Storyboard Artist to be working out of a trailer on location.
Storyboarding to me is more like a visual AD. Its like an illustrated one-liner. Others place it within writing, Pre-Vis, Art Dept. or Production Office(?) Sometimes they do get their own department classification. It depends on the Production.
I have many many crew lists, and the position of Storyboard Artist falls into many categories. Like everything else in film, there are no rules, so just like the characteristically bohemianesque job descriptions the Art Dept Artists define for themselves, likewise the Storyboard Artist.
Their undefinable department.