Because there is a long line of legal reasoning that says "natural born" means born in the USA, regardless of parentage.
"It is an established maxim that birth is a criterion of allegiance. Birth however derives its force sometimes from place and sometimes from parentage, but in general place is the most certain criterion; it is what applies in the United States; it will therefore be unnecessary to investigate any other.
James Madison, The Founders Constitution Volume 2, Article 1, Section 2, Clause 2, Document 6 (1789)
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Where in there does it state a "Natural Born Citizen" of the United States can also be born a British subject?
It states that in the United States, it is birth place that applies to citizenship:
“...but in general place is the most certain criterion; it is what applies in the United States; it will therefore be unnecessary to investigate any other.