Gold bars are technically "decentralized". No one controls the supply (e.g. there isn't a sovereignty that creates/destroys them), and I can technically just dig them up out of the ground.
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.