When people encounter an error with your product there is a high probability that they're already frustrated. And cutesy error messages just serve to trivialize their problems.
It's so prevalent in software, and I really don't understand why. As a user I tend to get rather annoyed when my apps apologize to me and say silly things like "oh noes!" or "whoopsie" or "your data got mixed up in a whirling tornado!"
Like, that shit's just not helpful.