To pay for a project like hyperloop someone would have to move 740,000 kg of gold to pay for it. Just moving that if it sunk or was stolen would destabalize whole economies. Now imagine projects that cost trillions.
It also would be slow to move, taking upward of a week if its across the globe (with huge risk for theft)
Now compare that to fiat which is mostly digital as you point out. It is nearly complete safe, and moved instantly.