In 80% of cases, it's because of authoritarianism. In 20%, it's because expecting someone to have a good sense of space to not poke an eye out with a drill, is much more reasonable than expecting the Charlatans to pick safe snake oils to sell.
No one ever buys random drugs they know nothing about, but things you know nothing about, are perfect Charlatan fields. Sad example:
Here in Greece, people literally tell each other medications to try, no doctor appointment required, as panaceas. It's not even for money, it's the same impulse that makes people share "super foods", and "mindfulness exercises". It's alienated, suffering people, with no real solution to their problems, making tiny religions around symbols. Just in this case, these symbols do liver damage.
So let's make it harder to trick people into injecting horse de-wormer or whatever. You have to at least be in the government to do that.