You know looking back when the Sims 4 was first designed or suggested within Maxis to be more online, I think some of that mentality stuck through development where if this game WAS a social online game where you play your own sim then in a sense you dictate your own sims behavior as YOU play vs how another player plays theres. I believe this will BE part of the Sims 5 if they go online with it too.
However once you removed that aspect and seclude the game in a closed world of premade sims - they suddenly have no personality because it wasn't planned that way. They leaned back on the emotion system to provide that, but by not adding emotional base states you could pick for your sims at least it basically made all your sims the same. All that traits do is provide environment buffs to those emotions.
I think they probably decided to remake the Sims 4 into a new type of Sims game, using a few old ideas but it has a lot of features that are very limited which is either by design to increase performance or they over relied on some features to replace older ones.
