Oh that's an easy one. But it's better to say our economy is based on oil: oil is exclusively traded in dollars.
That gives us a huge advantage over everyone else. And it's also the reason we can carry so much debt. As oil is traded in dollars, we will always be the safest currency to buy. Since oil, right now, is the worlds most valuable commodity.
I mean look around you. Oil is everywhere; pastics, food, drugs, cars, power - everywhere.