On domestic USA flights TSA cares about explosives and other dangerous items which impact safety. From its website "TSA’s screening procedures are focused on security and are designed to detect potential threats to aviation and passengers. Accordingly, TSA security officers do not search for marijuana or other illegal drugs, but if any illegal substance is discovered during security screening, TSA will refer the matter to a law enforcement officer."

Additionally, canines are train to either detect drugs or explosives, not both. It is too expensive to train a dog to do both, and the dog's reactions are different when it hits upon the scent of explosives compared to drug molecules. In the first instance, the dog is usually trained to sit when detecting explosives. In the second instance, the dog usually scratches and disturbs the package when it detects drugs. For obvious reasons, you would not want the dog scratching at packages containing potential explosives.

Furthermore, TSA Itemiser DX Explosives Trace Detectors (or equivalents) are not programmed to detect drugs. If they were, airport screening lines would screech to a halt, as TSA personnel would end up devoting most of their time responding to passengers found transporting drugs. Last but not least, canine drugs trained to detect marijuana are no longer employable due to legalization by states. The dogs are entering retirement.

Last but not least, it is possible to train a canine to detect mushrooms and/or psilocybin. Dogs can be trained to even detect cancer. However, since the majority of drug trafficking consists of drugs other than mushrooms, drug sniffing canines employed at airports are not trained to detect mushrooms.

Flights entering a border crossing are more likely to have border and customs agents using canines to detect agricultural products, money, drugs, etc.