Maybe they just didn't see the need or point to get married? In the end marriage in society is nothing but signing a bunch of papers and listening to a bunch of things that anyone who decides to be with someone for the rest of their lives should already know.

[Even in the small village near his volcano it was that way, and he doubts it was anything different in other places.]

Sometimes people give too much value to what's written on a paper. I've never seen them, but I doubt the birth certificates of my brothers and me state who our father was. So nothing states that we're brothers, but that doesn't change the fact that we are so.