In the past, rare objects were objects that were not widely circulated.
In the modern age, because it’s virtually free to reproduce any sort of cultural artifact, rare objects are objects that are widely circulated. This might seem like a paradox, but it actually makes sense: when you are exposed to so much information and cultural artifacts, there are only a few of those that can be widely circulated. This makes them rare to “possess.”
This is what might make NFTs valuable despite the fact that they can be reproduced fairly easily (e.g., if they’re a JPEG/MP3).