I think socially it's because addiction is stigmatized heavily. Despite having real psychological disorders that fuel it, addiction is still largely viewed as self-inflicted. Of course this stigma doesn't come out of nowhere - people who are addicts almost always devolve into dangerous and asocial behavior. If someone is an asshole who stole 50 bucks from you to shoot up, it's easy to think they're a heroin addict because they're an asshole. But really it's the other way around.