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.
I think you're right that they're thinking that way, and while you're right about how much easier it is to lose digital data, I think the real reason a backup solution should be implemented is because of how freaking easy it is.
If a fire came along and claimed all your photo albums, you're screwed. It was always possible to have a "backup" of all your photos buy simply buying two prints of them all and keeping the second copy somewhere else, but that's difficult and expensive.
Now there's really no excuse. For $69, anyone can back up 10GB of data for four years. And in fact, Crashplan is completely free if you're just backing up to an external drive, which is better than nothing.
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.
Ugh. I hate doing that. I've now implemented a policy where after the first visit to a new client, if they don't have a backup plan in place (which is 99% of them), I am certain to mention it. Frankly, I do this for my own piece of mind. I hate the thought of someone losing all their digital photos. But at least if I get that call down the road, they can't claim I never brought up the idea of backing up those precious memories...