So? Neither are most Americans.
You must be referring to 1st generation Americans, that is, the children of immigrants. If the children came here with their parents, or if the children were born before their parents acquired citizenship, then they are not the Posterity of We the People. If the immigrants become citizens (become We the People), then their future children will be the Posterity of We the People.
It's a harsh reality for some, and it may be an unfair luck of life's draw, but that's the way it is.