I don't write SimLit, but I make machinimas using sims, and write original novels. And if someone asks me why they're dark, the only response I can think of is a counter question: Why should they not be dark?
Not to belittle anyone else's preferences, but I personally don't get the appeal of "happy" stories. If there's no drama and no darkness, if everything is just perfect and nobody has any real problems, then... well, then you basically have The Sims 4. A fun game to play, but I don't see any creativity in recreating that in a story. It's the reason I stopped watching Good Witch, a show where nothing bad ever happens apart from some minimal conflict, and every character is nice. If you're a fan of that, I'm not judging, but I'm not, so I moved on, which is what I would recommend to anybody who doesn't like a particular story, instead of leaving unconstructive nasty comments.
I have a habit of enticing readers with fun and silliness and then hitting with the darkness hammer out of nowhere, and it's 100% on purpose. But you can usually tell by the blurbs of my stories that they're not exactly kid-friendly, and when writers even go as far as putting "no happy endings guaranteed" or similar content notes in theirs, then I have no compassion for people who still keep reading despite knowing that they're going to get offended.
