I've said this in many, many posts ..... my theory is that modders can enhance the game and "fix annoying things" because they actually play the game! As a player, they have gotten fed up with this illogical gameplay aspect or that irritating bug or want to tone down the over-zealousness of certain obsessions (which seems to be the devs priority when implementing "new" things).
I think EA hires people to work on a certain pack or gameplay element because they know how to do that particular task, but doesn't mean that person actually understands (or has ever played) TS4.
Plus, when they create new packs, I'm almost positive that they only test those features using the "new pack" and the basegame, never with every single EP, GP, SP, and kit installed because there is absolutely NO WAY that they wouldn't run into some of the issues players have if they had all packs installed when testing.
And, I feel like when they are "testing" they are simply just testing the mechanics of things, and not doing an actual play-through game session as if they are a player (such as the aging bug or the groundhog day bug), if they had created a new family and played with them for a few hours doing normal things that regular simmers typically do, they'd have discovered those same bugs as well.