To account for military bases and embassies abroad. ‘Under our jurisdiction’.
Not all places under American jurisdiction are in the 50 states. A substantial number of people live in these areas.
I think ...”and subject to the jurisdiction thereof,” may account for military bases, but from what I’ve read, the authors, in writings meant to clarify their intent, stated specifically that the 14th was NOT meant to grant citizenship to aliens. It’s primary purpose was to deal with slaves, the Indian nations, and diplomats.