that's not exactly true . . .
actually, according to the Whitney Museam of Art, many of those artists are considered to be Beat. I took a class on Beat Culture tought by David Quick, a Beat film maker. The class was taught from an interdisiplinary angle . . . allowing music, dance, theather, liteature, and visual art to all by taken into account. One of the texts was "Beat Culture and the New America 1950-1965" by Lisa Phillips.
The book addresses the issue of "Beat" meaning far more than just liteary works. After all, all of the origional Beats were interdisiplanry artists. Burroughs a film maker, Ginsberg a photographer, Kerouac a painter.
To name some of the Beat visual artists:
Jess (Robert Duncan's Lover)
There are many many more.