Why do we still have drugs in our streets?

Because the idea of destroying something like drugs at its source, something any organic chemistry student can make in his bathtub, is impossible. Even if it was possible, say, today. Since it doesn't eliminate demand there would still be a market encouraging a new generation to come up with a way to supply them. You might as well ask why you can't eliminate obesity by destroying all food, then sit back and watch a billion growers spring up and the successful ones become rich supplying others.