Photos like these give the mistaken impression that prohibition was effectively and uniformly enforced, and that alcohol itself was illegal.  People stocked up on alcohol, because possession and consumption were not illegal–only “manufacture, sale, or transportation.”  Additionally, you could produce alcohol for home consumption, and the federal government allowed for medical and sacramental use.  I’ve got a prescription slip for whiskey right here.

So, a better answer to the post’s question, “where did all the existing booze go?” would be: “into Americans.”