Even after marijuana legalization, many states still have thriving underground marijuana markets.
Many people who consume a lot of a substance over time will inevitably seek out cheaper sources. The sources complying with regulation and testing requirements, however light, will be more expensive than someone growing some plants in the back of a spare building somewhere.
The problem is exacerbated for more expensive and addictive drugs. The few people I’ve known who became addicted to opioids were spending significant amounts of money to maintain their habit and traveling to increasing dangerous situations to get better prices.
It’s optimistic to declare that everyone should have access to well tested drugs, but the reality is that it’s not a magic bullet. I also have some serious doubts about the narrative that increasing availability of drugs won’t contribute to more widespread addiction problems.