The problem is you are coming at this from a logical view. Logiaclly, it doesn't make sense to take drugs with all the risk ect...but most people who are doing things like that are not thinking logically. And it can be due to a lot of different factors...some have emotional problems, some peopel are thrill seekers and like the risk factor, some have had tragic events happen to them, some people can just have a bad chemical or hormonal imbalance that will start them down the wrong path.
You excluded all the prescription drug accidental overdoses are not being what you were talking about, but you are generalizing all drug users in the same category no matter what the reason/motoivation they had to start. Most people don't start out shooting up heroine. They start with something "less" risky like weed and progress up because it makes them feel good or numbes the emotional pain, quiets the voices, whatever.
If you never had peer pressure issues, self esteem issues, depression, ect you are very blessed. You are obviously a strong person able to deal with what life throws at you. Not every one can.