I sort of knew this, because of classes on teaching technical writing where we were urged to encourage students who were making manuals not to use clip art because it rarely represented any world other than the white, European, often male world of popular advertising prior to the 1960's.
Clip art is art that is in the public-domain.
An awful lot of it comes from old commercial advertising, from companies that have let the permission on the images lapse. For example, for a conference in which I am involved, we considered using a famous image by a great artist as our program logo. It seemed to perfectly fit our theme, and was just plain cool. What we wound up using was a little more diverse than a lot of clip art, but was still mostly white people. I was looking for images of women of color to put on the site.