User blog comment:DYBAD/Character Sheet/@comment-26362471-20171115224527/@comment-4867780-20171120030133

The Silver Sisters aren't so much needed as they are simply the best suited for this role, which is the natural extension of the many services and support they provide to citizens in Selforge City. And since Lawrence personally plays the fatherly side, it seems only natural that a member of the Selforge family would play the motherly one, which brings us back to the Sister's suitability.

As explained in the Legacy Heaven section, Bubble Worlds merge into Nexus worlds once the respective children living in them are "ready to broaden their horizons", as in leaving the nest and leading their own life, which is the perfect timing to meet others of their kind, a need they all feel eventually. Their Bubble Worlds aren't lost in the process, but instead harmoniously integrate a larger cosmology, preserving their individual past as they build their collective future.

A previous version of the LH had them live permanently separate in their own ever-expanding realm, but I couldn't shake the feeling that it was a bit lonely, especially in comparison to the family-centered Selforge City. Besides, since they have the same father and their mothers are sisters, not to mention their natural telempathic affinity, they all become very close very soon.

It's not so much they "can't" that they simply have no reason to. The Legacy Heaven is their world just like Selforge City is citizens'. As you said yourself said world is infinite both in scale and in what it has to offer, so the idea of something existing outside of it probably doesn't even come to mind, not to mention their immediate present is too happy to care in the first place.

Lawrence's extraplanar children eventually join Selforge City because their native world is both lacking and mortal, to moving on to a better one after a life well lived is the logical thing to do. The Legacy Heaven on the other hand is an immortal world ruled by  "perfect God" as you said yourself, so there doesn't seem to be much to gain in moving to Selforge City, does it ?