Don't forget, when Pets came out, it came with its own world dedicated to Pets. Sure, each lot was open. But pets didn't turn up in other worlds then either; Plus there was Sims 3's "You can only have so many packs activated at once" thing.
In Sims 4 I'm playing with all my packs activated at once so I'm going to resturaunts with Caleb while managing a child, a dog, and a brother with a job as a scientist. Hey, HE asked ME out. I do think it's a shame that every collection doesn't spawn in every world.
Nonetheless the argument that "No point because you won't be able to roam around" has limited effect, especially when you can now go to places like Granite Falls and Salvadorada that involve quite a bit of roaming.