For those that have not suffered through an addiction it may be difficult to understand, but leaving behind any addiction is incredibly hard, and people will hardly succeed on their first try, they will remain clean for some time and then they will go back to their addiction, so several tries are needed until the person can successfully leave their addiction behind and this is a process that can take years, with this in mind it does not surprise me at all that people simply give up and decide to keep themselves on the path of the addicted as this is the easiest path for them to take.
It's way difficult to let go of a bad habit than to cultivate a new one, in most cases the victim end up accepting their fate and live with it but then after several trial to quit an addiction some succeed and end the addiction while some give up because they find it really difficult to let go of the habit, for those who have been able to leave behind a certain addiction, they never wish to go back to it.
In case where the addict is finding it difficult to leave the habit it's very important to seek help but sadly some wouldn't want to for the fear of been judged or perceived as been a certain person or probably irresponsible, so they most likely want to stay with their addiction and leave with it hoping one day they would automatically stop which never happens. Professional help is always the best way to quickly get rid of addiction but other ways may work but will definitely be slow and incomplete as compared to when it's professional.