I think it's because 'civilization' is fundamentally about getting everyone to agree to abide by a common code of conduct. The restricted standards of behavior that follow offer various advantages over anarchy, albeit with fewer freedoms. Drugs are essentially anarchic because they inevitably lead to less constrained behavior and thought, which are threats to those standards and the willingness of others to play by the largely arbitrary set of rules that define and sustain civilized behavior in a fundamentally uncivilized world.