Why wouldn't there be another way in? How are the Feds going to check every single imported product? and the trucks, cars, etc that they travel in? You don't think the cartels can get their own drug sniffing dogs to see if their product and trucks pass the sniff test?
Under cover agents rarely bust drug kingpin type guys at the top... And even when they do, it's basically whack-a-mole. You knock 1 boss out of the game, another sees the opening/ opportunity and takes over.