Because in the real, non-theoretical bullshit world there are many undesirable social effect. For instance, dependence, irrational behavior, broken homes and death. Which necessarily involve other people and society as a whole. That's why. Sorry to be curt, but the idea an individual is independent of society approaches orthodox communism in lack of practical workability.

However I would argue that the current approach also breaks up homes, kills people and is generally irrational so that doesn't seem the way to go either.

Gore Vidal I think has the ultimate answer to the drug problem. Invent better drugs. Mitigate the harm. People will leave off the dangerous trash if there are better (and cheaper) alternatives.