Because no one ever had the idea to hide drugs in a cremation container and then cry foul when the drugs are discovered.
If people will hide drugs in their butts, they will hide drugs in just about any place else they can think of.
Personally, I don't even understand why drugs are illegal. If people want to poison themselves, let them, just don't show up at the ER, and expect the public to foot the bill, when something goes wrong.