There are a whole bunch of restrictions on moving gold across borders, and it's generally impractical to transact in gold for small transactions. You would have to not only carry the gold with you in useful increments, but also the appropriate assays to prove its value (which means you can't subdivide gold on the spot to make change).

That makes gold less useful as a currency than cocaine, ammunition or ram sticks.

Don't underestimate the power of incumbency. Gold has been something used as an exchange commodity far before currency. Its also impractical at scale so much less a threat to the general system.

Because governments own gold.