Because if simply making something illegal prevented it from happening, we wouldn't need jails.
So you make activities surrounding the prohibited activity illegal to make it harder to pull off. A truly dedicated criminal will get around all of this, but the more barriers you put up, the more impulsive idiots will be stopped. You won't eliminate the target crime, but you will reduce it. That's the theory of gun control -- you can't stop those who really, really want to kill someone else, but you can make it much more rare and reduce tragedies like suicide.
Right, because the drug laws have virtually stopped all illegal drug use! Well, I guess not - things are so lopsided that I can buy Crystal Meth faster and easier (no need to even get out of my car) than buying over-the-counter sudafed at the Pharmacy.
Sometimes making something illegal just means more people end up in jail without decreasing crime or making anyone safer or better off. It would be nice if a cocaine user could purchase his fix at the corner drug store so he knows he's getting a standard dose that hasn't been cut with drain cleaner or other noxious substances. And maybe it would be affordable enough that he wouldn't have to steal a car to pay for it.
Sure, some people will abuse the drug if it's readily available (just like there are alcholics), but those people are likely *already* abusing the drug through illicit purchases.