No one actually believes that. Not anyone with any kind of power anyways.
Watch "Reefer Madness" - it's reflective of the mindset these policymakers grew up with.
If you want to understand where that mindset ultimately came from, look at the history of cocaine prohibition - it boils down to racial politics.
It is only in modern times as I read more I learned the truth. Not everyone has time to educate themselves on these issues which many not concern them.