Nabu exists inside of the helmet of fate. Without a host body, he cannot manifest as Fate and cannot affect the physical world. His power is also dependent on the host. As stated by Klarion when Nabu possessed Wally, Fate was weak because Wally had no affinity with or belief in magic. Zatara also offered himself to fate by stating that his magic was at its peak and his body fully developed on which Nabu agreed and chose Zatara over Zatanna.