Welcome to the world as you only kinda know it, one devastated by an event called All the Apocalypses because, well, that's what happened. All the apocalypses one could imagine -- all the ones that weren't just automatic world killers, anyway, happened all at once on a cool spring day ten years ago: zombies, nukes, mutants, monsters, horrors from beyond ... all of 'em. Civilization lasted about ten minutes.
Those who survived had to adapt or die.
The Scooby Gang chose to adapt.