Quote from The Big D:
Because things aren't so very bad in terms of the utility of the dollar. One of the interesting properties of a fiat currency is that once it becomes popular, it can continue to be popular even in the face of policies that threaten to expand the currency supply.
Fact is, I can buy more with one dollar today than I could 2 years ago. And it's stuff I (and many other people) want - real estate in the US, electronic gadgets, cars etc. So from a practical point, my desire to hold dollars is higher than it was then - I can do more with them. In order for the dollar to slip seriously against other currencies, at least one of two things has to happen:
1) major inflation in the US
2) goods denominated in dollars are no longer things people want
Thus far, neither has happened.