Not going to be answered in a one liner but the short answer is historically it had about the highest density of value that was relatively easy transport, divid

Not going to be answered in a one liner but the short answer is historically it had about the highest density of value that was relatively easy transport, divide, and the inflationary properties though highly imperfect were less susceptible than that of factories or railroads. Humanity came to it empirically. Many people have suggested alternatives such as units of energy or baskets of goods but at the end of the day there is no perfect unit, dollars being backed by essentially largely threat of imprisonment if you don't pay your taxes and scarcity of how many the government decides to 'print' is yet another arbitrary choice of backing.