You’re missing the point. It’s about the Constitution. Where does the Constitution give the Federal Government authority to prohibit drugs? They had to have a Constitutional Amendment to ban alcohol, why not one for drugs?
The constitutional issues were raised and decided in the US Supreme Court. In brief, the Court held repeatedly that federal authority on those issues is based on the expansive view of the federal commerce clause that was adopted during the New Deal. That was after Prohibition. In addition, even when Congress has the power to act by statute, they may nevertheless propose a constitutional amendment so as to consult the states and secure both a political consensus and undisputed constitutional authority.