I see this (or rather, don't see it, lol) with my played households. I play a whole lot of Sims and households so that I can see them out and about to meet each other, but some never seem to appear anywhere and some who do, don't make sense.
I suspect it might be a game mechanic to prevent overload and save corruption, it probably works together with the culling system. As we've seen in the past, Sims games can get wonky, corrupted and bloated, so I think it's possibly that way for this reason. I wonder, though, if playing some for a certain length of time and interacting more with other specific Sims will get them integrated enough to become priority.