But if you decriminalize it, you can a) save a lot of money in the law enforcement sector, b) tax it, so you make even more money, both of which frees up cash to c) spend a lot more on prevention of rehabilitation.
Plus, as long as drug trade remains illegal, you have secondary problems such as bribery of officials, gang violence.
Maybe in the USA, in Europe (and probably other parts of the world, too) it is a different story.
AFAIK, we do not have anything like asset seizure in Germany. And the prison system over here is much less privatized / industrialized, so is not exactly a profit center.
A well-staffed, well-trained law enforcement apparatus is fairly expensive, too.
And consider that without proper treatment, widespread drug use incurs costs that do not show up on government budgets as such. Drug-related crime (theft, burglaries, etc.) comes to mind.
Plus, as long as drug trade remains illegal, you have secondary problems such as bribery of officials, gang violence.