almost all of the humans that have ever existed … exist right now
This is wildly untrue. The recent estimation of how many humans have existed is around 100 billion. Half of the total population of humans ever to exist was before the year 0, with the other half since then.
If you were able to answer that question, I think you would solve one of the eternal mysteries of humanity. We seem unable to escape the patterns that we keep repeating millennia after millennia.