I think part of it is that the game originally didn't have family trees, so there was no way to visually show them originally. But that doesn't explain not putting more clues and lore in game, like the missing Goth family cemetery. I love the few bits that are in there, like Johnny Zest's story. And that also doesn't explain the lack of lore and family trees for families in later expansions.
And unfortunately, having someone creating lore for the families would require resources and time that EA are probably not willing to allocate to Maxis. even if its just someone doing it when they would otherwise be twiddling their thumbs. From what I know of corporations, It would require a business case and approval by 3 layers of management with conflicting interests.
