Therefore the government understands that the war on drugs is likely to be unsuccessful, and we have to ask ourselves, why do they persist with it? A few reasons present themselves: Ideological motivations, they just don't like the drugs. The fact that if you terrify the population you can use that as means for greater political control and discipline. And the fact that Tobacco and Alcohol companies would likely suffer as a result of drug legalisation, like cannabis.
Lastly the CIA has been found to be involved in the drug trade on a vast scale. This is not a conspiracy, there are many well-documented books on this. They need large sums of untraceable money for clandestine operations, and drugs are an ideal source of this.
http://www.rand.org/pubs/monograph_reports/MR331.html
Noam Chomsky on the War on Drugs: https://www.youtube.com/watch?v=Q-JX0yXDlh8