More commonly, people are just buying an authenticated digital asset which confers no rights over the source material.
For example, An artist could sell unsigned instances of their digital art as .JPG files, and they could sell authenticated copies as NFT.
In both cases, you get a copy of the art. In neither case are you getting an implicit copyright to the work that would allow you to redistribute the work.
You could potentially create a real-world contract that ties ownership of the real-world asset to whoever holds the NFT, but that would introduce real-world legal issues like voiding the NFT in the event that it was transferred fraudulently and so on.
It seems a lot of people have been misled into believing that NFTs are something they're not. NFTs are like a special URL that points to some other content. You can trade that URL, but the content being pointed to still exists separately.
(And yes, it's just baffling that people pay for these NFTs when they really get nothing out of them.)
You can buy original animation cells from Disney movies, but try to resell that image commercially and you're probably gonna have a bad time.
AFAICT, NFTs are orthagonal to copyright. But it gets confusing because at least some sellers of NFTs are purposely trying to mislead, by omission if nothing else.