Why is Finn the human the only human?

Answer 1

Finn is the only human because during the war as in episode" i remember you"it shows the city after the battle Finns mom is in "the lich" so maybe she and his dad were taken to another universe it has not been fully released yet.

Answer 2

Finn is the only known human in the Land of Ooo because the Great Mushroom War caused the extinction of humans on Earth. Finn is believed to be the last surviving human, although the series does not provide a clear explanation for this.