Hence the ghosts

Both Shaggy and Scooby-Doo have nearly insatiable appetites, and are readily bribed by Scooby Snacks, as well as tendencies towards goofing off and cowardice. They justify their constant hunger by saying, "Being in a constant state of terror makes us constantly hungry!". However some believe the pairs constant eating is a subtle reference to munchies, a result of marijuana consumption.

Suddenly, it's all crystal clear. Literaly speaking.