A few other things, like interstate commerce, which the supreme court interprets as so broad as to cover a whole lot of things. The amount of shared laws are st

A few other things, like interstate commerce, which the supreme court interprets as so broad as to cover a whole lot of things. The amount of shared laws are staggering compared to the EU equivalent. There's also how medicare and social security are national, which makes a world of a difference when it comes to interstate mobility. The shared taxes that you mention are also huge: how different would Europe be if Germans were really on the hook for Greek budgetary issues? Look at this in practical terms: if the US states really resembled sovereigns, you'd have far wider differences in economic performance. Outcomes would diverge more widely if states were really doing things that were really all that different. We'd see incredibly different unemployment rates too. But in the US that can't really happen, because ultimately the federal government has everyone's back, and you don't see big, country threatening arguments about how California federal taxes are spent in other states: Those situations were possible back when the federal government was really weak, but the aftermath of the civil war took care of all of that. Along with the legislative differences, there's also a far more unified culture across states, if just for the shared language, and thus media. I can move from Missouri to California pretty easily: Better weather, higher taxes, but most of the rest stays about the same. Moving from an EU country to another is far more traumatic in comparison, which also makes each country be far more unique than US states.