The “why” of that question is basically due to humans being a VERY mobile species. All humans that are alive today are Homo sapiens sapiens (we add the second sapiens to show that we are a subspecies different even from archaic humans that lived thousands of years ago) because we do a LOT of mating with one another and that keeps us from getting too different.
As you might have guessed, technology has made it even easier for us to move around (and therefore, spread our genes), so that homogenizes (makes us more similar) us even further. For instance, this process of interbreeding was much slower in the past because we had to walk or sail or ride to mate with new people. Now, we can just pop on a plane and BAM (depending on how fast you are in the dating game), you can mix your genes with someone way across the world!