Likely because people like me cook it. Well I don't anymore, but I have, back when you could buy like 3 boxes a week and one store didnt keep track of another. And everytime there would be a big ass line, and I know that many people don't just all of a sudden have allergies.
However now they've figured out how to make meth out of Claritin. I swear junkies are fucking geniuses, if they'd only apply themselves to something positive..