While I don't know enough about European airport operations to give you a definitive answer, I'm going to guess that the incredibly high tariff/taxes we pay whenever flying out of a busy airport like London Heathrow [LHR] has something to do with how the Europeans manage security.

Besides, how many "domestic" flights are there for European nations? Even American airports require more stringent security on international flights, and pretty much every flight out of an European airport is an international flight.