But there are noble families and houses, and therefore lineages in Soul Society. Characters like Byakuya, Kyoraku, Ukitake, the Shiba siblings, Yourichi... they were all presumably born in the Soul Society.... where did their souls come from? Everyone else in Soul Society came there in the state they were when they died. That kid who was put in the canary from Chad's introduction Arc showed up as a kid in Soul Society. Rukia was apparently a toddler or baby when she and Hisana first wound up there.
So where the hell are the babies coming from? I do have my own theories, but if anyone has an outright explanation i'd be really happy to hear it.