The redneck strike again.

[Esquilax]

Now if this somebody according to our judgement does not exist what is the point in swearing to someone who doesn't exist.

Because it's a part of the vernacular you grew up with, and still functions as an expletive within that cultural context? Language is about conveying a message to other people, which swearing at or to a particular deity does. It's not about expressing literal belief in every word you utter. Haven't you ever heard of figurative language?