Europe is pretty prosperous. Some might even argue that in many European countries, folks are better off than the average American. Same's true for other countries defended by the U.S., such as Japan and South Korea. Germany and Japan, two of the countries most extensively occupied by the United States, are the third and fourth largest economies in the world. The only countries richer than them in that sense are China and the United States itself. The UK and France are right behind, at fifth and sixth place. Why, then, is the United States under any obligation to defend them? They are clearly financially capable of defending themselves. Is your stipulation that the the U.S. should be occupying every country with a smaller economy than the United States economy? Because that would be literally every country on the planet.

Let Germans defend Germany, Japanese defend Japan and Frenchmen defend France. Even ignoring any questions of imperialism, it just doesn't make sense for American soldiers to be stationed in any of those countries. It's not good for them, since you're never going to be able to learn to fend for yourself is someone's always taking care of you, and it's not good for the United States either, because it's left with the bill. No one benefits.