Because the problem is not the access to drugs. Sure, it's illegal, but you can find weed, heroin, cocaine, lsd, shrooms, anything you want provided you know the right sellers.

Only heroin is in the sweet spot of strong kick, immense pain relief and extremely cheap. People take cheap drugs because they're poor, not because they don't want to get their hands on pure colombian coke.

not everyone is a perfectly rational actor or in a sane mental state