The thing that is "non-fungible" is the token, not the actual art. The underlying digital asset is actually fungible with copies of it. With a painting, there are prints and reproductions of the Mona Lisa, but they are all fundamentally different in some way - the exact brush stroke texture, etc.
NFTs have this feeling of valuing the receipt, not the art. As if I walked into a gift shop at The Louvre and bought a Mona Lisa slick poster print, but then valued the gift store receipt more than the more-or-less completely replaceable poster.