Because the cost of treatment is a savings to society. Illicit drug use spurns break ins and lots of other forms of crime. The problem is that this country has always tried to work the supply side of the problem. It is basic economics, supply and demand. When we pay trillions to bring down supply, the price goes up and the crackhead has to hurt more people financially and physically to get what he percieves to need. Work on the demand side with treatment and we will go somewhere. No country at anytime has removed all drug use. But in the long run paying for treatment will be cheaper than the current system.
Better to never need treatment.
Now, is the enforcement being done correctly? Probably not ~ but it could be. The Chinese broke many addictions in the aftermath of the ascendancy of the Commies, and the Japanese were quick to disarm the pro-narcotics arguments after WWII.
I'm sure we could do something similar and much more humanely.
The other day there was an article on the net about how they can design a vaccine to eliminate addiction to cocaine, heroin, and other stuff. Use that stuff and no "treatment" is needed.