The ancients lived in a dog-eat-dog world. Life was rougher, more violent, and generally less survivable than it is today. For many people, one of the biggest concerns was leaving a lasting legacy, both in terms of ideas and DNA. The solution? Have lots of kids. Having a big family not only meant continuity, it also meant access to labor and the ability to survive more efficiently. Oh, and bonus: it was seen as a consequence of being in with God.
The same holds for nations, not just families. Why write a book like Exodus? For fun? Not so much. The writers wanted to keep these stories fresh in people's minds so that a national identity would develop. Looks like they succeeded—and then some.