The Constitution explicitly states that:
No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President
Since there is no one alive for whom the exception “at the time of the Adoption of this Constitution” applies, this can be condensed to:
No person except a natural born Citizen shall be eligible to the Office of President
I believe the founders wrote this into the Constitution because they knew that people have an inherent loyalty to where they were born. The place where John McCain was born is not even in the Jurisdiction of the United States anymore. Believe it or not, the place where John McCain was born is now being operated by China:
Negotiations toward a new settlement began in 1974, and resulted in the Torrijos-Carter Treaties. Signed by President of the United States Jimmy Carter and Omar Torrijos of Panama on September 7, 1977, this set in motion the process of handing the canal over to Panamanian control for free as long as Panama signed a treaty guaranteeing the permanent neutrality of the canal (Neutrality Treaty) and allowed the U.S. to come back anytime. Though controversial within the U.S., the treaty led to full Panamanian control effective at noon on December 31, 1999, and control of the canal was handed over to the Panama Canal Authority (ACP).
Before this handover, the government of Panama held an international bid to negotiate a 25-year contract for operation of the canal’s container shipping ports (chiefly two facilities at the Atlantic and Pacific outlets), which was won by the firm Hutchison Whampoa, a Hong Kong-based shipping concern whose owner, Li Ka Shing, is the wealthiest man in China.