Because some sort of magical ownership transfer to New York occurs when products are shipped to Europe?
Because when you spend money, you no longer have money.
Bill Gates or Stephen Spielberg bring millions into the the US, but that money isn't permanently theirs after they use it to buy things.
If Gates and Spielberg said that everything anyone else imported into the country was really bought and paid for by them, we'd look at them as if they were crazy ...
... which some of the secessionists actually were.