Because of legal tender laws and Gresham's law.

You can write a contract and specify payment in gold but if the other party doesn't pay and you sue them, you're required to accept payment in legal tender. You can't say "but I wanted gold!" Likewise, since you are forced to take legal tender if you want protection from the courts, according to Gresham's law, people would rather save their gold and pass off their paper currency to you. All of this serves to reduce the amount of gold in circulation as money and we get to the state of affairs like today where very few people could even pay in gold without first buying some with paper. That makes it hugely inconvenient and businesses don't want to shrink their market share. Remove the legal tender laws and have courts honor gold contracts and then you can make an argument. As it stands, when there is a truly free market, without government threats, the market prefers gold. You claim to be big on evidence so let's not ignore thousands of years of history.