Because fair use is an affirmative defense to copyright infringement. To use a fair use defense you have to admit your work is infringing, but argue that the infringement is justifiable.
Trying to defend the AI with fair use requires you to admit the AI itself is infringing, but justifiable, and by the doctrine of fair use, it is almost certainly not.
Only humans can hold copyrights. Your example would be a non-infringing work because it lacks direct copying. An AI doing the same would make an uncopyrightable work, with the AI itself being infinging if you tried the fair use defense.