The non-fungible nature of NFTs makes them valuable because they tend to be released in limited quantities:
Though one could see an NFT collection with 10,000 different models being minted and sold, they cannot be re-released once they are gone.
Traditional stocks can be duplicated almost infinitely, prompting people to sell the same share several times. However, NFTs are unique and cannot be reliably reproduced.
The unique factor of NFTs makes them more valuable with time, combined, of course, with the scarcity as mentioned above.