I've already suggested that it may have been an inculcated understanding due to the remoteness of America at the time and the assumption that people born here were born to natives of the land, given that travel from Europe took so long. Also, the Preamble to the Constitution refers to establishing the Constitution to secure liberty to "ourselves and our posterity," meaning the citizen people and their citizen children.
That said, from the argument that a born citizen is one who is born in the jurisdiction of the country (and not just on the soil), a legal alien would be under the jurisdiction of the country, but I think that an illegal alien is still under the jurisdiction of their homeland, so their child born on USA soil one day after illegally crossing the border should not be a citizen.
FWIW, the court in WKA said:
” Such allegiance and protection were mutual — as expressed in the maxim protectio trahit subjectionem, et subjectio protectionem — and were not restricted to natural-born subjects and naturalized subjects, or to those who had taken an oath of allegiance, but were predicable of aliens in amity so long as they were within the kingdom. Children, born in England, of such aliens were therefore natural-born subjects.”
Someone here illegally is not here “in amity” with the government, and thus would not give birth to NBCs. IMHO.
There is a specific tie of citizen parentage and place of birth in Vattel’s references which were very much known to the Founders. However the knot or blank so to speak in the debate about NBC as to a Constitutional requirement is that there is no specific wording that incorporates Vattel’s words exactly or some other corresponding expression into the requirement for POTUSA. My take is that with the records of deliberation of the Congress and the fact that Articles I and II address two different categories of citizen, the Founders intentionally wanted POTUSA to be a special kind of citizen. At the time their actions were in consonance with their words, I believe they thought so. I find it very easy to accept such even though my only brother who died on Okinawa in WWII and myself who also served in the Pacific would not qualify for POTUSA because of immigrant parents not being naturalized at our birth.