Durability is why not sand or platinum (or historical items like copper, silver, salt, etc)

Why does being physical matter? Because reality matters

Why has fiat lasted "so long"? That's a good question, I think largely it's a debt bubble that creates "perpetual" demand for the debt instruments to pay the prior debts. I think it's likely to collapse before too long and we shouldn't model the next monetary system on the worst aspects of the one that's collapsing now.