The fact is, addiction is a powerful, compelling disease of the human condition. It's a mental and a physical disease. There are biological, chemical, physiological, and psychological elements that the physicians, shrinks and the Ph.D's have been learning about for millennia.
The next fact is, not all addicts are intended to survive their disease. Thinking they will just because they're rich and famous is pure folly.
As some have already eloquently said, recovering from addiction takes active, physical and sometimes painful labor. It takes vigilance and it takes removing oneself from the equation, meaning it takes dropping ego, becoming humble, and accepting of a course of action that runs counterintuitively to one's own sense of priorities. It takes allowing other people and a much Higher Power to transform and heal oneself.
Hoffman either didn't do what was necessary or failed to allow those people to do what they could do. For him.
Either way, he's just another statistic. Not all addicts are meant to recover. He's simply one of them.