The record of Ether was a record of a new people, and to justify the authority of that new people, they needed to attach themselves to antiquity. Thus, the presence of the origin story of the world was part of their story, and they connected themselves to its beginning. That sacred beginning linked them to the biblical stories of creation, down to the time of the Tower of Babel. This confirms the comment about the record that Mormon made when discussing Mosiah’s translation and when reading that translation to his people:
“Now after Mosiah had finished translating these records, behold, it gave an account of the people who were destroyed, from the time that they were destroyed back to the building of the great tower, at the time the Lord confounded the language of the people and they were scattered abroad upon the face of all the earth, yea, and even from that time back until the creation of Adam” (Mosiah 28:17).
Because the knowledge in that verse was well-known, Moroni saw no reason to repeat it.
It would have been important to have what their record said, for it could have provided slightly different views of those events that would be useful to religious historians. However, assuming that Moroni had that record in translation, it is plausible that Mosiah had already translated it in conformance with the scriptures he knew, perhaps in the same way that the citations of Isaiah in the Book of Mormon come from the King James Version that Joseph knew, rather than from a retranslation of those texts.