Fair question. And you know the answer: It doesn't.
And then, let me pose this one to you: "Where in the Constitution (excluding Amendments - I want this to be the Framers' view) does it say that children born in the United States to non-citizen parents are automatically citizens?
To make this simple, I'll stipulate: it's nowhere in there.
The Constitution nowhere sets any conditions on citizenship by birth. None. Citizenship is equally well conferred by lineage birth as it is by any other way, and certainly mores than citizenship by the accidental or incidental location of birth.
As Emmerich Vattel said in The Law of Nations: "I say, that, in order to be of the country, it is necessary that a person be born of a father who is a citizen; for, if he is born there of a foreigner, it will be only the place of his birth, and not his country."
Nevertheless, it's not in the Constitution. Therefore it is a debatable proposition, and one that has seen much controversy. It's not easily settled, but when it is it will be settled in favor of the expression of the plenary power of Congress to legislate in the name of the Sovereign, We the People.
Which is why both Cruz and Rubio are ineligible.
The Constitution does not specifically say that children born to US citizens outside the US are automatically citizens - true.
But the Constitution DOES say who gets to decide that in Article 1 Section 8 by specifically enumerating Congress with establishing the rules of naturalization. Further in the very first Acts of Congress, the naturalization act of 1790 was passed that set the precedence for children born to US citizens overseas shall be natural born citizens. Now some try and argue that the act of 1790 was repealed and replaced and that is true, but those acts have also be repealed and replaced. Several times in fact. Until you get to the current law as expressed in USC Title 8 Section 1401 - Nationals and Citizens of United States at birth.