That's because we (as an agrarian society) have constructed an idea of “what a human is” in opposition to animals, but with the advance of AI maybe we'll realize that we are much closer to animals (especially other mammals) than we're comfortable to admit (and no, I'm not vegan).
And, to be clear, it doesn't make humans that special. Other animals have created art that expresses their sincere internal experiences and desires.
If I'm being perfectly honest with myself and you ask me what makes humans deserving of special moral consideration, I would have to say nothing. I think an elephant, gorilla, octopus probably all deserve the same consideration. I give more consideration to humans because I'm human, and what makes us human isn't art. A person with severe brain damage and the inability to understand or produce language or art is still human.
ChatGPT mirrors this, producing output that feels generic precisely because it's trying to predict what's the most likely response. That's in direct opposition to interesting creative works.
You can force it off the beaten track a little by adding more details to your prompt, but it will still try and fit the best curve through those additional points.
Until now, if you ask "what makes humans different from animals" many responses could be in the line of "generating art, poetry, music, synthesise ideas"..
Well, now it seems somewhat clear that THAT part of human activity can be performend by something non-human.
(There is still a lot more to humans of course; I agree with you on that point)
Yes -- it is only reassembling stuff humans in the past have done -- but so is most of human activities in these areas. Very little humans produce is not inspired by input from others. Art, music and ideas reverberate back and forth between humans and only slowly change over the decades.