Here's why:
* Percy is constantly facing threats from mythical creatures: Monsters, gods, and demigods are the primary antagonists throughout the series.
* He must learn to control his powers: Percy's own demigod status and the growing strength of his powers pose both an internal and external conflict.
* The overarching plot revolves around a battle between good and evil: The series is a battle between the Olympian gods and the forces of chaos, with Percy caught in the middle.
While there are some instances of person vs. person conflict (like Percy's rivalry with Luke Castellan), the core struggle revolves around Percy's ability to navigate a world of gods, monsters, and his own demigod heritage.