Because the ocean doesn't know where Te Fiti is. That island, the source of creation, can only be found by a god. It needs Maui to be there in order to find the island to even get the heart back.
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Moana is the daughter of a tribal chief on a remote island. Her village never ventures beyond the reef because of the inherent danger of the ocean. However, the island is dying, and the only solution requires venturing across the sea.
However, certain European countries faced copyright issues with a trademarked name, leading to the film being titled Vaiana in those regions. An example of this is Italy, where the name Moana was trademarked by a travel company, prompting the film to be released as Oceania in that country.