With all due respect, we don't know that. Knowing this as a fact would mean provably demonstrating the degree of instability at any instant of the universe, essentially its whole trajectory in a potentially infinite (and at least incredibly high) dimensional space.
It's just as possible that for a given person, molecules could have changed somewhere or even "bigger" events could have happened one or more times, and their existence would have remained. Automatically assuming a chaos of infinite possibilities with completely divergent universe is potentially reducing the complexity of the world and its countless smaller stabilizing feedback loops that locally do reduce divergences. Otherwise we'd never do anything for fear of spontaneously exploding or other funny things.
