Easy. So patriots like Thomas Paine, who was foreign born to parents who never set foot in the US, could become President.
If “natural born citizen” is synonomous with “natural born subject,” then no exception is needed for Thomas Paine.
All those who were citizens at the time the Constitution was adopted had parents who were not US citizens, nor were any of them born on US soil, because the US didn’t exist until then! It was created by adoption of the Constitution! So everyone was a naturalized citizen.
So again, you are refuted.
No, it’s because no one could have been POTUS if they hadn’t. No one would have qualified.