The short answer is that is the way they made it. They being people in power corrupted by greed and megalomania.

Originally only gold and silver were money. You could trade for money or dig it out of the ground. If you brought your gold to a government mint it would be made into coins of known purity and weight. Government coins were a trusted unit for trade. However, gold money limits what a government can spend due to a natural physical limit on supply. The primary benefit of gold money is nobody can conjure 100,000 tonnes ($6.43T) out of thin air for their own benefit. That puts the government on more equal footing with the people.