You're right of course, but it's way riskier to take physical possession of commodities and it's harder to move them. These are things crypto is better at.
For cash, the advantage is that crypto is independent of government action and global. The crash of the Russian currency is an example of why this is important. Crypto is obviously more volatile than USD, so there's a tradeoff depending on the government backed currency's stability.
It's also harder to move similar amounts of wealth in cash or commodities (you could have self custody of a wallet with a billion dollars in it, can't really move a billion dollars in cash or gold).
These advantages are real and people are relying on them: https://news.ycombinator.com/item?id=29518181
