Its the exact same reason why you work hard when you own a part of the company you work for, vs. one you just get paid at. You have a sense of entitlement (ownwership) that you're working hard for your own (and others') good. The US economy revolves around the US dollar in very substantial ways. Nobody wants to see themselves or everyone else fail, so 'a dollar' stays relatively static within the bubble of comfort. Outside the bubble of comfort where USD isn't the universal trade tender, one must be realistic and pragmatic about things. That's why there are thousands of people who go to work daily just making sure the relative 'value' of currency remains in check along with adjusting based on the ability for the backing governments to pay their debt obligations. This is also why its a hell of a lot more volatile to trade in currencies than it is to go to the grocery store to discover the price of milk.
Ther 'loss of confidence' inside the bubble almost always follows the collapse of banks who are no longer able to honor the depositors. When it becomes systematic (think Greese if they still had their own currency for instance), people immediately attempt to pull their money out of the banks. Pulling money out of banks means there's less credit for businesses to draw from. Less credit means either companies collapse or raise their prices. Collapse means less value in the bank, whereas higher prices means there's more inflation on prices, which yet again forces people to invest in tangables which then 'hold their value' better than a nose diving currency.
I'm sure there's a ton I've missed and some that's just plain wrong, but it gives an idea on how 'currencies collapse'