The problem is you cannot test economic policy in the states. Social policy, absolutely. But if you try socialism in one state and its neighbor is a tax haven, business interests will just jump over the border and you will see wealth flight. It is too easy to move between states when you have money. It is much, much more complicated to move between countries.
this fact is what makes the experiment perfect. If people all move from one place because of poor policy it will give that state the incentive to adjust their policy. People should have the freedom to chose the goverment that is most inline with their values. Forcing someone to stay somewhere and pay taxes to a say unjust war or something they are morally opposed to seem to be a terrible existence.
The problem is that there may be policies which work well if the population participates in them consistently, but which fall apart if everybody gets to pick and choose. The whole point of government is to do these things.
But if most the people agree that the policies are good and best combination of all the other states wouldn't they pay those taxes voluntarily instead of having to be coursed into paying? That is democracy right? otherwise it seems kinda selective as to whose votes mater the most.
That works if and only if the best individual choice is also the best group choice. This isn't always true, see the Prisoners' Dilemma.
For example, perhaps society benefits when you tax the wealthy to help the poor. But given the choice, the wealthy will move to where their taxes are lower, so that plan works poorly if you let individual states pick and choose.
Or perhaps society benefits when you don't tax the wealthy to help the poor, and make the poor pull themselves up by their bootstraps. But the poor will move to a state with more aid, again defeating the plan.
Obviously there are differences between the states right now, so this isn't a total obstacle. But it really only works to the extent that there is a lack of mobility between the states (people tend to want to stay where they are), or on issues where it doesn't matter.
Wealth inequality is the perfect example. It is in the best interests of society to keep inequality under control - extreme inequality, like we have today, has negative social and economic implications. But if you are on top, with all the money in the world, why would you ever voluntarily give your money away?
Like the other response said, its prisoners dilemma. It is in the best interests of the rich to pay for honestly a majority stake in the society because they are the ones prospering the most from it. But if any one person of the ownership class can avoid paying, they have an incredible competitive advantage over their peers they will obviously exploit.
Independently, it is never in your best interests to give your money away in taxes. It is always in your best interests to have everyone else pay them but you.
This is why you cannot test any number of policies - basic income, progressive taxation, universal healthcare, amongst many more - because if you are not getting more out of these policies than you put in you have the instant ability to flee from paying for them.
