The division into three scrolls is not a printer's convenience. The first ends precisely where the imperial line begins, and that boundary is the argument the book was commissioned to make. Everything in the upper scroll happens among kami: the deities who come into being at the parting of heaven and earth, the pair who churn the brine and make the islands, the sun goddess born from a washed eye, the storm brother expelled from heaven, the ruler of Izumo who yields his country. The moment a sovereign takes the throne in the plain of Yamato, the scroll is over.
Read that way, the upper scroll is a title deed. It establishes that Amaterasu is the ancestor, that the regalia she gives her grandson come from her hand, and that the earthly powers who held the land before agreed to hand it over rather than being merely conquered. Nothing in it is incidental to descent. Even the long strings of names that a modern reader skips are doing the work, because a genealogy is the form the claim has to take.
That also explains an absence. There is no creation from nothing, no legislating deity, no moral code delivered from the sky. The scroll does not tell anyone how to live. It tells them where the ruling house came from, and it is the later reading of the Kojiki as a religious book, rather than the compilation itself, that turned this material into something resembling scripture.