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!"
You should read up more on the opioid crisis and fentanyl specifically. Poverty and "lack of economic opportunity" is not a new thing. We're dealing with something entirely different and these armchair expert comments and opinions don't help things.
The crack epidemic in the 80s wasn't fentanyl. Whatever its divergent chemical properties, I'd rather address the reason people start using it in the first place. If we don't fix that, then even a magic rehab pill that instantly cures addiction won't have any meaningful impact.
As for "poverty and lack of opportunity" not being a new thing, no shit. But we've seen a hollowing out of the middle class over the past few decades. So a lot of newly impoverished people who grew up middle class, or are else seeing their quality of life decline through no fault of their own. That is relatively new on the scale that we're seeing it.
And I say this as someone who's immediate family has had some past issues with fentanyl addiction, so it's anecdotal, but I'm hardly just speaking from my armchair here.
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!"