I know this tread is very old, but I have noticed it's mostly down to the lack of depth.
Like many people, I went from one game to another as they released, thinking the next insallment was 'better'...and then after I received my copy of Ultimate Collection and watching some Sims 2 creators, I gave Sims 2 a go again. And found myself playing for hours and hours again, like I did when I was a teen.
I asked myself, "Why do I not feel like this about Sims 4?"
And the writer in me gave me a clear answer:
In Sims 2, playful Sims get in the bath and play ship, grouchy Sims are mean to others simply because they can be. The memory system makes Sims randomly and spontaniously react to stuff in their past, like cry over the death of a loved one years afterwards. And if you tell them to do something out of character (like directing a sloppy sim to clean or a shy sim to talk to someone) they will look at the player, go "please don't make me do this :(" and do the task looking very uncomforable. I can turn autonomy on and my Sims will act according to personality and interests.
Cindy from Pleasant Sims even created a system that allows you to use the Zodiacs entroduced in Nightlife to choose a Sim's aspiration, which I love because it adds a little bit of random to my games. I don't really need to tell them what to do and I most certainly don't need to spend all my mental energy thinking for them.
You see a slight decline in this depth in Sims 3, but it's barely noticable. The trait system isn't perfect, but Sims still act in character...most of the time at least. The open world and story progression does make up for this, and it's fun to check in with households now and then to see what's changed.
Then comes Sims 4. An empty shell of it's predecessors. A large part of why it's so boring and tiring to play Sims 4 comapared to 2 is because the Sims are all the same, with a spot of "personality" painted over. The traits don't actually do anything other than affect emotions (a system which itself is very flawed and broken). They have added sentiments and fears recently, which does seem to help a tiny bit. The story progession is a nice touch, but it's oveshadowed by the game costantly generating new randomised townies en masse with no way to turn it off without mods.
Sims 2 feels like watching a story unfold, with you subtly directing here and there. Sims 4 is like a having to explain a very simple concept to a very dense coworker 400 times and then they still don't understand, leading to you having to do everything in the end.