The reason why most governments don't legalize all drugs despite the argument that they can tax it and such, therefore benefitting the economy is because that's not realistically possible. People often draw parallels with tobacco being taxed; it's not possible to do this with most drugs however. Tobacco is rather difficult and expensive to be grown privately and made into cigarettes by an individual person. Therefore people just buy it at the stores and it's grown by tobacco farmers with all the resources, equipment, and so on. With marijuana, meth, heroin, crack, etc this isn't the case. These drugs can be grown, processed, made and so forth in just about any home. If you legalize weed for example, people will just grow it because it's relatively easy to do, no need for stores, taxes, and the like.
edit: spelling