Because USD aren't legal tender in the Eurozone, so you couldn't actually buy anything with them. You could change your euros into pounds and move to Britain, except your job, family, friends, and so forth are still back home (and once everyone gets that idea, it's not going to work for a million reasons).
Now if you're talking about people with investments in EUR who could switch that over to USD, well yeah, the exchange rate doesn't seem to have crashed. Hm, can you short a currency somehow?