because the American Gov't gave away that authority to the Fed Reserve (which is not a gov't organization)
The Federal Reserve was created on December 23, 1913, when President Woodrow Wilson signed the Federal Reserve Act into law
Congress has the right to modify the Federal Reserve Act which is why we need to control both houses and the WH
Truth is, at best this "payroll tax" suspension would give you a 7% raise - it is just your SSI with-holdings