I think most people figure that they don't have a backup (excluding insurance) for other tangible items in their lives, so they apply the same logic, to computer data. And while it's true that there are many catastrophes that can completely and instantly devastate tangible property and possessions, it doesn't take an otherwise (or typically) significant event to completely wipe computer data.

A 12" drop can kill a drive. And that's an easy to imagine scenario that needs only a momentary loose grip or a quick snag of a USB or power cable as someone or even a pet walks by.

Unfortunately I've had the opportunity (misfortune?) to tell friends "you should have had a backup" more often than helping them actually implement a backup strategy.