I've got a two note range. My wife's family sings harmony just as easy as breathing. But there's a couple of reasons for that:
A. They're a musical family. When you grow up with, it's easier.
B. Church. Used to, local churches had singing schools. The Free Methodist church closest to my wife's home was noted many years ago for not allowing families to sit with each other. You were seated with sopranos, altos, baritones or basses, depending on what you sing.
Shape notes, mostly out of Heavenly Highways.
It's a shame, but most of those folks have died out and the young people aren't as interested.