I will suggest instead that drugs are illegal for two reasons: Moralization, which is basically randomness applied to ethics; and because it allows our justice system to pick and choose who to imprison.
A third reason is that generally speaking, they cause a lot of pain and suffering. I’m against prohibition, but dismissing it entirely as moralizing and social control misses the very real dangers of drug abuse, and their attendant social costs. It’s just that the only thing worse than the harm caused by drugs, is the harm caused by drugs and their prohibition. Legalizing drugs is just step 1 though. Step 2 has to be a massive overhaul of treatment for addicts and the mentally ill.