Because she's one of the main heroines...except she might as well be a tertiary support character.
Hypothetically, take her completely out of the story after the teleport incident and resolve the ED arc with Roxy in THIS arc.
Does a lack of Sylphie change the narrative? Not one bit. That means she presently serves no purpose in the narrative.
Ariel is going to call Rudeus. Not Sylphie, she's done in the story. Arguably, the only reason Sylphie even needed to come back into the story was to serve as a lazy plot bridge to connect Rudeus with Ariel. Which his cousin does anyway. She's redundant. The fact that she's very boring and obedient means there's no means for her to change the dynamics within the series.