I never quite understood Americans' desire to forgo empire and "focus on home."

America's withdrawal from global affairs will create a vacuum that someone else will fill. This someone else will, very likely, be a lot more malignant an empire builder than America ever was.

Who is to say that this new power won't subsequently set its sights on America?

I might not agree with the American empire but I can see the rationale. It is to prevent any power from becoming influential enough to seriously threaten the American mainland.