There are only a few cryptocurrencies that have any significant adoption, and fungibility is fundamental to that value. If I exchange a bitcoin for $N, that validates the value of all the other bitcoins. And folks exchange large numbers of bitcoins for $N every day.
By definition, every non-fungible token is its own unique snowflake. There are new snowflakes made every day. Few ever trade, and the ones that do are probably wash trades.
Yeah come on it should be pretty simple to explain why Bitcoin has value. It's because there is demand for it, for whatever reason. A "scarcity theory of value" feels like someone hamfisted the labor theory of value into the cryptocurrency space. No, even if labor is scarce and precious then making beanie babies for which there is basically zero demand (I assume) is still producing no value.
I'd say most of the value of Bitcoin comes from the ability to do payments that are not possible in the current legal system. Some of them may be illegal. Some of them might be genuinely useful like in El Salvador.
And certain NFTs also have scarcity built in. Both NFTs and crypto coins can choose the level of scarcity they want, and all of the value for that scarcity is determined by whatever people believe it to be. NFTs are the exact same thing as crypto coins.
Consider this statement: Scarcity is fundamental to any store of value. There are an infinite number of potential cryptocurrencies.