Those who are alive have genes that have survived billions of generations due to our ancestors' habitual vigilance for predators, prey and mates. We are hardwired to scan the world for those, whatever the channel. Our predecessors who got bored too quickly by massively repetitive scenery were less likely to become our ancestors.
Tell that to all the descendants of Genghis Khan....
If we go back in history far enough: yes, of course we will realize that X% of humanity is a descendant of person Y. But that is just a rather obvious consequence of the fact that genetic signals can travel through a genepool pretty quickly. Nevermind that we are talking about ancestry, not any particular genetic signals. It's just that Genghis Khan, King John and Charlemagne are rather prominent ancestors, and that their geneanalogy is documented well. I'm pretty sure we would find similar results with most people who still had decendants by, let's say, 1750. Since then, humanity went through a phase of tremendous population growth, which by definition expands the set of descendants of any of our ancestors. Most humans that have ever existed are alive today. Their ancestry must come from somewhere.