Here's a link to the Register's editorial on President Bush's promise to provide more money and advice/training, etc. to Mexico to help the government there crack down on narcotraffickers. If lavishing money on enforcement could stop drug use and trafficking, it would have happened long ago. If the definition of insanity is to keep doing the same old thing but expect different results, this is a classic case.
Two reasons. Prohibition creates a huge price premium for illicit drugs -- most experts think about 10 times what the price would be absent prohibition -- which is not enough to keep some users from paying the prices, and a heckuva profit margin. As long as there are some people willing to pay the price, there will be unscrupulous people willing to go after those inflated drug-war-premium profits. Bouts of conspicious enforcement tend to weed out the less skilled operators, reducing competition for those who are really good at it. Some progress!
Then there's the Iron Law of Prohibition, which I've mentioned before. Conspicuous enforcement leads traffickers to focus on more easily concealed, higher-margin products -- so we get less "soft" drugs like marijuana and more "hard" drugs like heroin and cocaine. Again, it's worked that way every time, and to expect different results from doing more of the same is ... well, you get the picture.
The real problem is that the enforcers and traffickers have a symbiotic relationship. Without prohibition the traffickers would have such low profits it might not be worth it, and the enforcers would have to find honest work.