I feel like you're trying to find a more concrete reason than is necessary - the fact that it is unique can be valuable on its own. I can go buy a reproduction

I feel like you're trying to find a more concrete reason than is necessary - the fact that it is unique can be valuable on its own. I can go buy a reproduction Picasso that looks identical to the original and from an art standpoint it's effective equivalent, but it's still worth nothing compared to a "real" one. The fact that the original was painted by Picasso is what provides it so much value, even though who painted a particular piece of art is not a physical part of the art itself. A unique NFT effectively provides value in the same way, it is valuable as long as people care about who/what issued it and thus want to be able to say they own that unique one. And while it's notable that digital art can be duplicated perfectly and physical art cannot, I don't personally think that matters all that much. People who are buying a Picasso aren't doing so just for the art, they're doing it so they can say they own a Picasso. In the same way you're not buying an NFT for the art itself, you're buying it because it's uniquely identifiable in a way you and others care about.