They do, sometimes. It depends on barriers to entry. It's hard to ban marijuana or alcohol, because it's so easy to make. Relatively true for meth as well. But

They do, sometimes. It depends on barriers to entry. It's hard to ban marijuana or alcohol, because it's so easy to make. Relatively true for meth as well. But you don't see many people doing quaaludes anymore because it's not easy to make. Additionally, a ban like this doesn't have to be 100% effective to be worth the trouble. The time frame is also important. Saying "bans don't work" is absurdly reductionist.