That is due to economic mechanisms that restore equilibrium. A very important mechanism is labor market flexibility. If there are lay-offs in one part of the US, people tend to move to other, more buoyant regions. A further mechanism that contributes to restoring equilibrium in the US is income transfers by the federal government. To be honest, similar redistribution mechanisms to absorb shocks on an interregional basis are less developed in Europe.
Labor mobility is limited across Europe, and even within countries.
Fiscal transfers between countries would be politically difficult to implement and they lack public support. In addition, the current European Commission budget is very small in comparison to the US federal budget, and certainly too small for significant fiscal transfers. Instead, our system relies much more on the intertemporal absorption of shocks within individual countries. This works through automatic stabilizers in national budgets, which are large in comparison with US state budgets. According to the so-called Stability and Growth pact, member states should have fiscal surpluses in boom years in order to allow for possible deficits during economic downturns.