It is amazing how alive Sims 3 sims feel, even with all the limitations of an eleven year old game. I think one of the marks of a truly good game is how well it makes you care about the characters, even as you know they're not real and it doesn't matter what you do to them. Not to make this about bashing Sims 4, but the few times I've played, I've been sort of absorbed in the moment but have never had the slightest interest in going back for another session with the same sim. Now I don't bother to try: I just name them "test case" and randomize their traits, and I wind up in exactly the same place.
I remember my first Sims 3 sims quite clearly, not only their lives but the way I perceived them. If I still had those user data folders, I'd probably go back and check them out some time. It's not that I spend time contemplating their personalities and imagining how they feel about their lives, it just kind of happens. And that's why I keep coming back to the game.