Here's why:
* No mention in the books: J.K. Rowling never explicitly stated that Helga Hufflepuff's ghost existed.
* Other founders have ghosts: The ghosts of the other Hogwarts founders (Godric Gryffindor, Salazar Slytherin, and Rowena Ravenclaw) are present at the castle.
* Possible reasons: It's possible that Helga Hufflepuff's ghost simply isn't present at Hogwarts. She might have chosen to pass on in a different way or remain in another realm.
While it's fun to speculate, the information provided in the books doesn't support the existence of Helga Hufflepuff's ghost.