I think 'self-harm' is being used too broadly here. That term usually refers to people who harm themselves as a way of coping, and is a sign of more serious psychological problems. However someone who hurts themselves may not necessarily fall into that group, just as someone who drinks may not be an alcoholic. That doesn't mean self-harm is never worth worrying about, just as the existence of responsible drinkers doesn't mean alcoholism is never worth worrying about.
As to why people freak out about self-harm more than drugs or alcohol, it's because they don't understand it. Most people have a strong desire to avoid pain, so they don't understand how anyone could deliberately hurt themselves (whether it be self-harm, masochism, knife play, aesthetic body modification, or any other reason I may have missed).