I think human art is only defined in the context of humans (or similarly developed entities). The best paintings of human history won't have much artistic value outside human viewers such as pets or possibly even aliens (should they exist).

Conversely, we can only understand, for example, animals' art to the very limited extent we understand animals: that is, we don't really have much of a clue if an animal doodling with pebbles in the sand is making art and trying to make a beautiful (to him) arrangement of stones, or just moving pebbles around for fun.

If we saw alien art, would we even know if it's art or something else? We would have to know the alien species, their culture, and how they live in order to even make educated guesses.

It might be that the concept of art likely carries over across species but we can only see in other species' art what is similar in our species.

If a monkey draws a monkey face we can understand that because we have similar eyes and we could draw a human face. But if we don't share even basic senses with some entity we can't possibly understand what visual art might be for them.

So, a machine producing "art" is something we can only understand as reflections to what a human would produce. We judge the machine's output by as if it was created by a human. But because we know it's not human made but just a compilation of random values and preset rules, it lacks the context we might call the human element and becomes void to us.

At a minimal scale the "human element" could even just be the story of the person who painted the image, even if the image is simple and not skillfully very refined. But if his paintings are a testament to how he after tenuous hardships ended up living on a small island and started painting, then we can reflect back to his story by looking at his pictures and try to see what parts of his life might be captured on the canvas.