Ideally, currency should just exist and people should pick it based on how well it suits their needs. National currencies are fine in an environment where most

Ideally, currency should just exist and people should pick it based on how well it suits their needs. National currencies are fine in an environment where most economies are mostly domestic, because the effect of import and export on currency availability and foreign currency exchange is miniscule. This was the environment when these national currency schemes were devised. In a globalized economy, every country has trade imbalances which means they all need to have foreign currency reserves, adjust their reserves when economics change on the ground and adjust interest rates, this is a massive overhead for every country in the world that doesn't need to exist.