Because for many (most?) addicts drug use starts as a coping mechanism. So the question is what are people coping with that's so bad it causes them to turn to drugs, despite all the readily available information about how bad they are? Lack of economic opportunity among the lower classes is a big one.

There are certainly other causes of addiction, but at the end of the day a lot of rehab programs fail because even if the addict gets clean, they're immediately sent back to the living situation that got them addicted in the first place. This is especially true for the homeless, "congrats you're free of heroin, now go back to your tent under the bridge where you live in constant fear of rape from other drugged-out homeless people and no employer will touch you! You're free!"