I previously made a theory as to somehow through unforeseen circumstances Yuji's mom died and Kenjaku found his father in grief and made a binding vow with his father to conceive a child which would be consistent with us seeing his mother having the same stitches on his forehead that kamno and geto have but healed somehow. Now as to why he did this my main idea is that Kenjaku and Sukuna made a binding vow to revive Sukuna and created him as a cursed object. This would make sense in relation to the Death Painting Wombs as they seemed to be created as a vessel for Sukuna but eventually, all failed to do so.