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".
Singapore doesn’t seem to have a huge drug problem, and they went all in on what you call “the war on drugs”. So maybe there is a winning strategy there, at least for Singapore?
1) there will always be demand for drugs, thus fulfiling that demand legally will reduce the harm caused by low-quality/tainted drugs, the side-effects of the underground drug trade (violence, weapons smuggling and gangs branching out into other crime) and can be offset by taxation (as long as the total price including tax is still lower than what the black market offers).
2) existing users who are addicted can't just quit cold-turkey, they will need a certain dose of the original drug or an adequate medical substitute for a period of time - that dose can be lowered over time as part of a recovery/rehab program but cutting those users off forcibly will hurt them physically and force them to seek out black-market sources.
Even without that, the process for getting clean is not easy. You can't walk into a hospital with withdrawals and expect to be admitted -- unless it is alcohol or benzos they won't do anything at all for you.
We should as a society stop saying 'they chose their fate' and instead give them options for a softer landing when they decide that they had enough.