Why do you think fairy tales begin with the phrase 'Once Upon a Time'?

I think that it is a way for the writer to let you know that this is a story, and not true, even though we are going to tell it as though it were true. It's similar to saying "once, a long time ago" ... and is just a way of saying it where the listener or reader knows that what we're launching in to here is fantasy, not reality.