Because while gold bars might be technically decentralized, that doesn't mean they are practically decentralized too. In fact, gold bars are probably 99% centralized in corporate and governmental vaults, neither of which are openly audited. Also; lots of 'paper gold' is sold, suppressing the value of it.
Crypto can be secured and transported much more cheaply and easily - as opposed to gold, where many people would rather own/trade a receipt for gold kept in a secured vault.