Because interfaces require situation-specific reasoning that purely expressive art can be imitated without. And frankly, I think the slick looking images that AI spews out from actual artists' munged up work is a far cry from being equivalent. Like interfaces, the people who wouldn't pay an artist or designer to begin with probably don't care enough about the quality of the art or interface for it to matter. Cookie cutter applications and stock images are going to suffer, but developers making basic crud functionality or simple API interactions will suffer soon after.