Because it is mimicking human input. Effectively you are getting a mixture of many pieces of artwork that humans made distilled down into some sloppy new one that was made without feeling, purpose or skill and that can be described by its prompt, a few kilobytes at best. Original human art can only be approximated but never captured with 100% fidelity regardless of the bitrate, that is what makes it unique to begin with. Even an imitation by another human (some of which can be very good) could stir you in the exact same way but they'd be copies, not original works.
Anyway, this gets hairy quickly, that's why I chose to illustrate with a crappy recording of a magnificent piece that still captures that feeling - for me - whereas many others would likely disagree. Art is made by its creator because they want to and because they can, not because they are regurgitating output based on a multitude of inputs and a prompt.
Paint me a Sistine Chapel is going to yield different results no matter how many times you would give that same prompt to Michelangelo depending on his mood, what happened recently, what he ate and his health as well as the season. That AI will produce the same result over and over again from the same prompt. It is a mechanistic transformation, not an original work, it reduces the input, it does not expand on it, it does not add its own feelings to it.