Then imagine you've taken something that infinitely transforms your world and makes you feel amazing to such a degree that's absolutely unimaginable to someone who hasn't tried some of these substances ("better than sex" is an extreme understatement). All your pain is gone, all your misery forgotten, amazing vistas open before you, all is good.
After that brief stay in paradise, you come crashing down and back in to your miserable life. Everything is as before, your passion is back, only you now know it's possible for you to get back to that exalted state, to escape that prison of a life you've been living in, even if for a moment.
Is it evidence of a disease for someone who's gone through this to desperately want to experience that escape again? To be free and feel great?
Is the cause of such a desire -- the misery, the poverty, the hopelessness, the fantastic way these substances make you feel and the escape they offer -- can all that be justly summed up in the word "disease", or is there something else going on?
Many addicts I've known and seen come from stable environments and it isbecause of their incessant drug use that they end up poor, homeless and degraded.
Of course some addicts definitely start in "crushing poverty" but many do not. It affects people from every background, every social strata.
If your hypothesis were correct than the housing and support programs would work. They don't.
Treatment is elusive because it isn't possible in the traditional sense.
The phenomenon is the exploitation of two things: the vulnerability of humans to chemicals, and the inability of communities to protect their free spaces from the outcomes of humans abusing chemicals.
Free spaces are a sacrifice zone. Legalisation of addictive drugs, at scale, will destroy what we value as community.
If we legalised all drugs, we would need to protect our communities, not abandon our communities to people destroying themselves.
I believe addiction is a disease, but treatment is elusive. Some have no desire to stop. There are also frequently other mental or emotional problems. Just feeding addiction's most obvious symptom (by continual drug use) does nothing - see this article.
I doubt whether there will ever be a neat and tidy "solution". Eventually the healthy part of society will have enough and some action will be taken, like prohibition or war on drugs, etc. Then it will wax the other way again.