Addiction. Addictions don't care about how smart the person is, what they know or even how well informed they are about the dangers of what they are doing to themselves. Do cancers or " the flu" not happen just because someone may know alot about it? Some can quit with knowledge but many can't. They have to want to quit then get the proper treatment to help them with their disease process. It can be done.
Addiction. Same reason that well-informed and highly educated people can drink too much, overeat and become overweight and/or obese, abuse drugs and engage in other unhealthy and unhelpful behaviors. Many of these folks have unresolved childhood trauma and attachment issues while others are simply physical dependent on their substance-of-choice and it is hard to quit.