>What makes you think that is the case?

The fact that the incompleteness is easily shown. Whether you look at the overall number of people who have used a certain drug vs the number of people who ruined their life with it, or at individual cases that didnt develop that way.

I also wouldnt go as far as your reframing of "certain drugs have certain results" to "certain people have no problem with certain drugs". Given the high impact of a users socioeconomic background for how he develops i think you can look at the way people use a drug / perform addictive behavior and find pattern for the people most and least negatively effected. Its not that different from finding the group of people that get morbidly obese through overeating or get into debt for gambling. You can often tell them by their planed behavior and lack of self reflection and self control in that behavior.

You know certain stuff is addictive and messes with your self control. If this fact sneaks up on you or you planed to solve that with overconfidence in your self control you are pretty likely pretty screwed. And the people with the most problems here are those that keep going and accelerating despite knowing they have no more breaks.

Its also not something that should take you long to realize looking at how you stick to your planed consumption and your ability to react to that. Think days for some people. With the next group at risk those that fall asleep at the wheel. And so on. While i wouldnt risk it with cocaine and the likes, i think arguing that its impossible ignores a bunch of very successful people. You could argue that this just means those are the people who manage to stop on their own after it escalates and then stay away from it, but this ignores the people who somehow have a tendency for not accelerating routines they cant recover from too far. If i had to guess those might the the people who trust themselves the least, i am pretty sure you are unlikely to land in such an arrangement by luck.

We can of course argue about the difficulty of balancing on this spectrum for certain drugs, but pretending this is a matter of "there could be" and not the history of actual individuals sounds a lot like a self fulfilling prophecy. Especially since it glosses over the people who stopped their usage for other reasons then having lost control (health, happiness ...)

Individuals, their predisposition to certain drugs and addictions, their risk profile and their behavior are just very different.