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.
You are deliberately glossing over the reality of the situation. Yes, I deliberately exaggerated it, but not by much.
The South was producing 70-80% of all export value. Exports are paid for by imports. Trade approximately balances over time. Yes, the merchants and industrialists of New York had garnered enough support in Congress to get laws passed that favored the use of their City for the vast bulk of the European trade, and they had acquired favorable conditions, both through congress and some occurring naturally, to monopolize the shipping of US exports to Europe as well as the coastal packet trade.
But they were taking bigger bites out of the Southern apple than the South would have put up with had they a choice.
The South was effectively paying for 70-80% of the imports, with much of that value being siphoned off by the people who were controlling the import/export trade.