Indeed, most people don't move. I won't be able to put my finger on it now, but several years ago, I read a statistic that a huge percentage of people are born, live, and die within a 20 to 30 mile radius. College attendance, military service, sightseeing vacations and travel for work all bring us out of that circle for a time, but we tend to return to it afterward.
My brother has his radius whittled down to about 10 miles. I, on the other hand, live about 800 miles from where I was born and grew up, and have no expectation of returning, but then, I always was the black sheep of the family.
Well, it not only implies that black people don't move, but white people DO move. Either that or it assumes that the ratio of black to white was much greater than it likely was. It also assumes a lot of other things.
What's mysterious about people staying in the same place for generations? My people have been in Iowa since the Civil War too.