Quite simply because it is possible. If the Everett interpretation of quantum mechanics is correct, and there is no known reason why it couldn't be even if it

Quite simply because it is possible. If the Everett interpretation of quantum mechanics is correct, and there is no known reason why it couldn't be even if it might always remain untestable, every time there is a quantum level interaction (erroneously called an "observation") the wave function does not collapse at all as in the Copenhagen interpretation but instead remains in the superpostion of all possible outcomes. That whole mess propagates and some of those possibilities interact with some of the possibilities of what is described by the wave function of other quantum interactions ad infinitum. This is called the "many worlds hypothesis." There aren't really many worlds in any usual sense although all the possibilities continue to exist and exercise their influence on every other possibility that has an intersecting causal tree. The net of all this is that everything that can happen does. It's just our perception that gives the illusion of a single path through the whole schmere. The philosophical ramifications of that world are enormous. It puts paid once and for all the whole "free will" nonsense but it leaves a certain despondence because it doesn't much matter what you do in terms of the perception you are "experiencing." You do everything you can possibly do and experience all possible joys and sufferings as consequences. Ever wonder why you continue to exist despite the ridiculous odds of you having survived all the close calls you can remember. Because it's possible and from your perceptual tree you don't see the ones in which you didn't. In my personal case I find this observation the closest thing to a proof of many worlds that I could need to be persuaded of its correctness. It just ain't possible that I'm still here after all this time and all those things that happened yet... Also there are a nearly infinite number of point's in your past from which you live forever if from the initial state of the universe a path of possibility exists to that state. Whether or not such a path must exist is where it gets hazy for me. It doesn't mean however that any of your descendents from "this" particular perceptual point will necessarily persist ad nauseum even if there were points in the past where they could.