An NFT is an autograph linked to bits that commonly represent images, videos or tweets. The NFT is created on a blockchain (decentralized db). As soon as the blockchain network is shut down or its members disappear, or the owner of the autograph forgets their digital wallet password, the NFT is gone.
The only thing that you’re buying and selling that’s not fungible is the blockchain-specific token. The video, image, etc, can be copied many times by anyone.