Because foreign law doesn't matter when it comes to whom the US considers to be or not to be a citizen.
False
It DOES matter. Read what the drafters of the 14th Amendment (the Senate Judiciary Committee) have to say about that:
"The provision is, that all persons born in the United States, and subject to the jurisdiction thereof, are citizens. That means subject to the complete jurisdiction thereof. What do we mean by complete jurisdiction thereof? Not owing allegiance to anybody else. That is what it means."Congressional Globe
Okay, then here's my case: My great-grandfather immigrated from Italy 100 years ago. My grandfather was born here before my great grandfather became a US citizen. Under Italian law, Italian citizenship was passed to my grandfather, who never renounced it (since he didn't even know he had it), to my mother, and to me. If I come up with the documentation of that chain--birth records, marriage records, naturalization papers--I can get an Italian passport. So, I'm born in the US. My parents were born in the US. My grandparents were all born in the US. But under Italian law, I'm an Italian citizen. Are you saying that I'm not a natural born US citizen?