The self-expression of art is in its creation, not in its final product. Yes, the self-expression usually results in differences in the final product - if you hired 2 people to make a painting off of the same detailed description, they would be different paintings, largely because of differences in self-expression. However, if you were to, for example, hire 2 different artists to make perfect copies of the same painting, to the point where they're indistinguishable from each other, the self-expression would still come in when one artist uses a different tool than the other, or starts with a different base color. The methods both still result in an identical final product, and so the product doesn't showcase their unique self-expression, but the creation is separate, and unique to the artist.
Notably, you, the person who asked them to make the art, contributed nothing but a prompt. Yes, that prompt resulted in nice pictures that you wanted, but the self-expression - the thing that makes it art - was entirely someone else's. It's their art, they just made it for you. AI "art" is the same thing, except it's made by a lifeless computer devoid of self expression. So, it's still your nice picture, but there's no self-expression at all, and so it's not art.
Hmm... That's a good point. I've been focused on whether or not the final product is art, and it's definitely not, because again, it's created by a machine that can't express itself, but it's true that the prompt itself may be considered art. The thing that can make a shovel into art isn't the shovel itself, but the self-expression coming out in the artist's idea to look at it in a way that elevates it to become art. But there's a difference between elevating something mundane into art, and randomly declaring things to be "art." Marcel Duchamp's work is art, but I think most would agree that people who ridicule it by saying things like "Okay, then this pencil's called 'Mr. Writey' now - it's art!" are not creating art. Maybe the only real difference is intent? I'll have to think on this one, but I appreciate your insight!