Because there would be a masive depression, a financial crisis, or both. You've got causality wrong in your "twin deficit" theorizing. 1/

A fiscal deficit is a necessary response to a serious trade deficit, on demand grounds (you won't be in full employment if your consumers are buying everyone else's goods, unless the state runs a deficit to restore incomes)... 2/

and financial fragility grounds (the financial system will create fragile private securities to fulfill the huge demand for securities that mercantilist trade counterparties help provoke). /fin