"The United States is a federation of self-governing states within which there are open borders and free trade"
The point is, it is rather easy for a wealthy person to move to a low tax state. I've known several retired people who picked a particular state as their new home, once they had retired. And corporations do this all the time -- they move from one state to another so they can be in the state where they can pay the least in taxes. This limits our ability in the USA to impose the taxes necessary for a healthy set of social safety programs. Let's think about this as engineers -- if we see a system of pipes, and there is a leak in one of the pipes, what would we do? In the short term, we would plug that leak. In the long-term, we would ask "Can this system be streamlined so there are less pipes, and therefore less things that can leak?" That line of reasoning has lead me to conclude that we should amend the Constitution in the USA, abolishing all local and state governments, and concentrating all power in the national government. That would allow the government to impose the taxes necessary to give the country the kind of broad social programs that European countries already enjoy.