If the Louvre decided to sell an NFT of the Mona Lisa, nothing prevents them from generating another NFT (or a few thousand) of it as well. Same thing for an NFT pointing to an image at some URL and containing a hash of that image, it doesn't actually generate ownership of that art in any legally meaningful (or enforceable) way.

Digital ownership only works for things that 100% stay in the digital realm, and even then it's not a guarantee.