Because there is no safe, above-ground supply chain for those drugs with actual quality control and penalties for contaminated supply.
> there’s people taking drugs where they don’t even know what’s in the drugs
It's not like those addicts have a choice of walking into a pharmacy and being able to buy a safe drug manufactured by a reputable pharma company with a datasheet of exactly what's in it.
Both of these problems are caused by the stupid "war on drugs".