How to use "So it goes"

It was interesting at Billy's anniversary party when Trout was making a story up off of the top of his head when he was talking to a women named Maggie. "Trout was making this up as he went along. 'Just before the casket is closed, the mourners sprinkle parsley and paprika on the deceased.' So it goes" (p.171). Trout was lying to the woman about this book which didn't exist, where characters who never existed died and they had a funeral. But, Billy still had to say "so it goes" when trout said "the deceased".  How could something that was never living die? Was billy commenting on the idea that would die because it would never be made into a book? Billy has to "so it goes" whenever anything mentions death, but that is because a life never truly dies. It just gets replayed by time travel. The moment when this happens will be replayed in Billy's mind, but is the character in a book that doesn't exist really dead if it was never alive to begin with? I don't think it really died, so Billy should not have to say "so it goes".