It's not what we attempt to achieve that matters but what we actually achieve. Drug law enforcement initially set out to protect people from problems involving drugs by making certain substances legally unavailable. However, enforcement of such drug laws can lead users to switch to other, more dangerous drugs. It leads to huge, uncounted costs to society from criminalisation, from the spread of infectuous diseases by behaviour encouraged by illegality. It expands opportunities for violent criminals.
Drug money allows gangs to recruit more criminals.
It is their life blood. So much money to be made with things that are illegal will corrupt law enforcement and all security branches of the government. Abusers will shy away from seeking help for fear of getting arrested. Users will lack safety advice and controlled product quality. Black market dealers sell to buyers of all ages, and they often sell many kinds of drugs.