The one thing to know about addiction is that everyone is different. If you think that it's easy not to be an addict, you may just lack empathy.
It's easy for you to quit smoking? Good for you. But it's very clearly not the case for most people. Feels to me a bit like saying "it's easy to be rich, you just have to be born in a rich family like me".
Circular logic. What do you think "discipline" is? It's just a word people use to describe people with strong rational, goal-oriented behavior, with ability to follow through over long periods of time.
This is also called "executive functioning" and it's wholly dependent on your brain chemistry, i.e. the thing that is malfunctioning in addiction.
So yes, they lack "discipline" because discipline is also physiological, it's not just (and maybe not even primarily) a reflection of the person's character. By treating them as separate you're engaging in mind-body dualism.
It's easy for you to quit smoking? Good for you. But it's very clearly not the case for most people. Feels to me a bit like saying "it's easy to be rich, you just have to be born in a rich family like me".