Because they'll find a way to have drugs whether we want them or not.
What we need is to show them the way, why and how to use or not use drugs.
Drugs are not to the problem, the problem is the mind. Thinking drugs are the problem is an incredibly narrowminded way of looking at the world and it will never work.
Buddhism explains all of this. If drugs are an escape (or done with a bad intention), we can't let people not escape, we need to understand what they're escaping from and lead them in a better way.