Americans (and not just the US) think they still have the influence on the world they had in the 1980s when their economies were a much larger proportion of the global economy. Americans have no idea what the world looks like from Asia which contains most of the world's population and generates a third of global GDP.

They also came to realize building, operating and maintaining a military force is extremely expensive. Free healthcare, 7 weeks of vacation, 36 hr work weeks, unemployment benefits, subsidized housing, etc etc is all great when you don’t have the financial burden of protecting your home.