Addiction is a disease. It's a disease that lays dormant in people until the right conditions are meant, and then a switch flips. What you said is similar to saying "I don't know how people can just get skin cancer. I've never had skin cancer, and I don't plan on getting it and I spend lots of time in the sun." https://www.nejm.org/doi/full/10.1056/NEJMra1511480 While I agree that addictions are a disease, the difference between addictions and other diseases is that addictions are preventable. If a person never took the substance in the first place they wouldn’t have an addiction. The same cannot be said for most other diseases. Now, there are valid uses of some of these substances, and ignorance and information could lead into it, but again the root cause is that the substance was taken. This difference is likely what causes the different mindsets against addictions vs. other diseases.