If you illegally sell $10 worth of drugs, then record your front store of selling $10 of Tylenol (that you don't actually buy or sell) then the $10 of drug money has been fully taxed.
There's taxes from the supply chain of Tylenol that don't get paid when it's $10 in illegal drugs that are actually sold. Much like Bitcoin, with a black market you can only tax at the egress points when the money needs to touch a regulated financial entity whereas an all-legal system can tax any time money changes hands.