I do not understand why someone would do this shit.
Because of the cascade of neurotransmitters it unleashes, which cause intense pleasure. Also, the manufacturing process for pretty much all refined drugs is awful and involves lots of organic solvents and things that explode and other ugly stuff. Even legal drugs like SSRIs. In fact, pharmaceutical companies are responsible for tons of Superfund sites.
They keep doing it because it eventually burns out your neurotransmitters to the point you develop anhedonia and are unable to experience any kind of pleasure at all without the drug, and eventually, need it just to feel normal. That anhedonic phase is pretty ugly with the stimulant hard drugs like cocaine and meth, and can last years.
Also, it seems people inclined to addiction experience drugs in a different way than those who can take it or leave it, or who actually just hate drugs. A lot of normal people don't experience euphoria from drugs at all, but just feel weird or at the very least realize they're doing something really bad for them that they don't want to repeat.