I cannot speak for others but I know that in my case, it was only when I stopped that I began to notice just how freely available most things are. It seemed like eveywhere I went I would bump into people who wanted to use and I did manage to keep my friends AND to stop - but I think at that point, even friends who used had recognized that I had almost killed myself so much harm did I render to my own body.
I am also prepared to suggest that getting straight is the easy bit. It's recognizing that until you stop, your entire life revolves around obtaining your drug of choice. So many people stop and ask the very reasonable question 'what's the upside in my doing this?'
So I had taken the precaution of keeping a diary. It's now 25 years later and I still cannot read that diary. If tempted, I just open it at random and within a page, I am reminded of just how miserable I was.