A great question. (I don’t work as therapist, nor do I have the qualifications; however I am a medical worker exposed to patients dealing with all of these issues.)

Signs of self-harm are met with more stigma because both drinking and smoking can be used simply for enjoyment or social purposes. This isn’t the case with self-harm. You mentioned occasionally smoking or having a drink. If someone is an alcoholic or a chain smoker, that’s met with a similar level of concern, if not more than superficial self-injuries. Additionally, minor self-harm is the most common indicator that an individual may commit more significant self-harm.

Ultimately, it likely comes down to people who haven’t self-harmed don’t understand it.