Here's why:
* Omniscient means "all-knowing".
* An omniscient narrator has access to the thoughts, feelings, and motivations of *all* the characters in the story.
* They can move freely between characters' perspectives, giving readers a complete picture of the events and the inner world of the characters.
Example:
"Elizabeth Bennet was not a woman to be easily persuaded. Though Mr. Darcy had proposed, she felt a deep-seated resentment towards his arrogance. She knew, however, that he harbored a secret admiration for her, even if he wouldn't admit it."
In this example, the narrator knows Elizabeth's thoughts and feelings, as well as Mr. Darcy's secret admiration. This is a hallmark of an omniscient narrator.