Because that would imply that there do not exist any persons who are born on US soil and have citizen parents.
And that is not a representation of reality.
The existing diagram is.
What shall we call ourselves, those of us born in the country to two citizen parents, us nameless folk in that hole you created? I've just wracked my poor little brain and can't come up with a thing, bless my heart. /s
Forgive those of us with intelligences inferior to yours, for thinking that you just didn't know what a Venn diagram was.