I call this the "anthropic principle of fiction", based loosely on the actual anthropic principle.
Loosely speaking, the anthropic principle states that it's no coincidence that we happen to be found in a state where sentient life exists, because that's the only state in which there are people who can ask "how did we get to a place where sentient life exists?" (For example, there's no sense in which we're lucky to be born on Earth instead of Mars or Mercury, because, out of the three, we only could have been born on Earth.)
Similarly, suppose some event happensset of events happen in a fictional world that warrants a book. That book will cover the experiences of some people. Some people in this fictional event didn't survive long enough for there to be a whole book about them. So it's no coincidence that the ones you're reading about in the book you picked off the shelf were the ones who survived for most of the duration of the eventlong enough to have their exploits covered by a book.