Addicts generally know at some level that they are addicted. The core problem is more that addicts can't break the compulsive behavior without a significant intervention.
Your subconscious and instinctual drives have ludicrous control over your conscious thought. Anyone who knows an intelligent drug addict knows the lengths of rationalization that a conscious brain will wrap itself into in order to justify a fix.
Talking to a clever, articulate addict can be downright scary Their ability to rationalize isn't just limited to fooling themselves and downplaying the problem to others. It's quite a surprise to hear someone justify their habit so persuasively that you have to go back and reevaluate whether they're right.
(Of course, another part of this is that a really self-aware addict is especially likely to see the motivators of their behavior. If they're consciously self-medicating, even harmfully, "just quit" is a crappy answer.)