Heroin is one of the few drugs where "once is enough". Many people go through phases experimenting with drugs in specific party contexts (e.g. raves) but that doesn't carry over to daily life. The people who carry it over are the ones looking for an escape as parent describes.

Edit: And of course it doesn't require that they have obviously impoverished hopeless lives. Part of the illness of our society is the huge numbers of depressed/lonely/etc middle class people who otherwise seem to have a life "on track"