Because "citizens" includes both "naturalized" and "natural born" citizens.
Why is the word "natural" inserted? It is a matter of allegiance.
And allegiance to the Framers was understood as a matter of the circumstances of birth. They never expressed any doubt as to the allegiance of someone born within the U.S. That's why jus soli was the rule of citizenship -- one was deemed to owe allegiance to the sovereign of the realm within which one was born. That was the essence of "natural law."
Note that native born is not the same as natural born.
As to those who are domestic-born, they are the same.
"Under our Constitution, a naturalized citizen stands on an equal footing with the native citizen in all respects save that of eligibility to the Presidency." Luria v. United States, 231 U.S. 9 (1913)
Thus, per the U.S. Supreme Court, the only persons excluded from the Presidency are "naturalized" persons. Native-born persons are citizens by birth under the common law and the 14th Amendment and need no naturalization. Ergo, they are "natural born citizens."
IOW, the 14th amendment says more than "All persons born or naturalized in the United States are citizens of the United States and of the State wherein they reside."