The reason is that addiction is exists as both a physical disease and a mental illness. This is one of the areas where the body and mind overlap more than most. Humans usually begin engaging in addictive behaviour as a means of coping with some unresolved trauma often suffered in their youth, but sometimes as an adult. While most people only think of addiction in relation to substance abuse (beer, cigarettes, hard drugs) it can manifest in many different ways.
I suffer from chronic addiction myself. What this means is that I will -always- find something to be addicted to, whether beer, marijuana, video-games, sex, social media, anything. And it is all an attempt to reduce anxiety brought on in relation to past traumas. These traumas can never be erased, and anxiety is simply a part of every day life, therefore I can never truly be cured of Addictive Behaviour itself. It's just a part of who I am.
Instead, I channel that behaviour into more constructive activities, such as exercise, low-stress, high activity jobs, reading, music, aka "Things that are good for me. " And as I've done this, my need to use alcohol has decreased. I have to limit my time only (and that's been difficult, since I've been out of work due to an injury) but I've been using that to post on support forums, and build skills to help me cope with life's worries.
Ultimately, addiction cannot be cured, just rerouted into a non-destructive activity. It may be necessary to make major changes in a persons life (in my case, changing my career, leaving an abusive ex, abandoning my former bong-buddies, moving towns, changing diet, leaving most social media...) you get the point.
But if the addict wants Happiness instead of Convenience, recovery is possible.
Addicts are like alcoholics, never recovered but always recovering. The addiction prints itself so deeply on every aspect of an addicts life that behavioural triggers are often as powerful as the chemical ones.
One slip, one opportunity and most recovering addicts will slide into temptation.
My addiction was overcome through the strength and love of my family. Many addicts don't have that support system and that makes it much harder for them to start treatment and much easier to bsckslid.