And we need federal workers to approve beer labels...why?
Short answer: because Congress, in its infinite wisdom, has said that it's a crime to sell beer otherwise. Until that law is repealed, no brewer is going to take a chance on violating it.
Longer answer: because the federal excise tax on beer is a function of the amount of beer in a can or bottle and the amount of alcohol in the beer. (Notice that it's the Treasury Department that's in charge of enforcement.)