Short answer is: Well, there isn't one, despite people in this thread saying otherwise. I have a few theories.
At first, it was because there was no genealogy in the base game, therefore there was physical limitations AGAINST giving family trees to premades.
As for ones introduced since genealogy was implemented;
1. I have a feeling it was partially as error prevention. For those who haven't played The Sims 2, or at least since release, one thing you need to know is that whilst premades had vast and interesting genealogy, many of the sims on the tree not only took up space in the save file - which could cause issues and corruption later on due to there being a hard save file size limit, but many of the ancestors were just straight broken in The Sims 2. If you tried to resurrect them, some could permanently damage your save files.
A lot of clean saves fix this by removing anything before the previous generation.
The Sims 3 on the other hand, the family trees with deceased sims were still there, but way less vast, and all of the dead sims were, in turn, resurrectable with no issues. I think a couple had missing traits, but nothing game breaking like TS2.
2. Maxis figured people didn't care about premades, and therefore didn't invest any time into fleshing them out - not a priority for them. Growing up, I was the only kid I knew in my friend group who liked the premades in The Sims. Everyone else made people they knew IRL, and to me that's boring. Why would I make people I know IRL when I speak to them every day anyway? I want to escape in The Sims, make new people with new stories, or explore the stories Maxis made for us. That said, growing up I always liked art and creative writing. This may be a huge contributing factor.
As soon as I came on the forums, I realised there was a lot of others like me. But I'd assume that the vast majority of the player base don't know anything about or care about premades. I don't think any of my friends back in the day could name a single premade, and I know my sister can't name any from The Sims 4 and she has 1300hrs in the game. She just doesn't care about them. I don't think the people on these forums make up what The Sims franchise fan base is actually like, generally speaking.
Edit: As for why I believe they changed the twin's mother's name - it's censored on the forums, likely because of the first 3 letters of it. it's not even pronounced like that, but I do think that this is the reasoning - so people could discuss the sim and her name without it being censored.
