Well, that's a hard question. Why do people eat fast food, drink alcohol, smoke cigarettes or eat medicines prescribed by the doctor? All the above are also really harmful to the body. There is a lot of drugs out there, and a lot of them are not that harmful to the body, it's often the user's fault that these things happen. Overdoses and of course the lifestyle itself.

Alcohol is more harmful to the body itself than for example heroine, but the life that often comes with it is the worst thing. I've met people who have been addicted for 10-15 years but have a normal life, dinner with friends and family, good parents and a regular job. But I've also met people that have gone from a normal life to a life in misery, days without food, not showering and selling their soul for more drugs.