Carriers are good for force projection beyond a nation's connected landmass. The Americans are in a fairly unique position that they must have a large navy to get to anywhere "interesting". Unlike Europe or Russia or China who can literally just march to the Middle East or Africa if they have to (although Africa is a long trek for the Chinese...) Among other "interesting" countries, European NATO members share borders with Syria, Iraq and Iran. The Middle East and North Africa are within tactical striking range of European airbases. America is like an island on the other side of the world...