There isn't enough gold to use as a common currency.

As I understand it, all the gold that has ever been mined would fit in a cube the size of a baseball diamond.

https://www.businessinsider.com/warren-buffetts-lesson-on-go...

Nixon was responsible for ending the silver standard.

https://www.usmoneyreserve.com/news/executive-insights/when-...

https://en.wikipedia.org/wiki/Silver_standard#United_States