Why isn't the American government/NASA harvesting raw materials from the main asteroid belt?

Some small metallic asteroids, with a diameter of 1 mile, are projected to contain up to $20 trillion US dollars worth of industrial and precious metals. That would pay off our debts pretty quickly. I know it costs too much to bring it back to Earth at present, but who says we have to bring it here? Why not keep it on the moon? A space bank perhaps? Our money could actually be backed by something real again.