My guess is that they have a limited pool of resources that they can spend which is limited by things like the system requirements, the game itself and their budget (time, assigned people, other resources).
Their system requirements are very low, so I agree with what others are saying that it feels unnecessary to design a pack around those requirements because a computer with those specs are most likely going to run into problems after a short while as their save file grows larger. But that's probably not good business for them because they want to appeal to a large player base.
The game itself seems to load and remove neighborhoods when you travel so I don't think that matters too much, what probably matters is how much the game needs to index and keep track of, save files seem to grow very large over time and a lot can contain thousands of build/buy items so a lot can probably take up a lot of resources.
When it comes to their budget I think they are also pretty limited, they are already making quite a lot of content every year and they probably have to spend a lot of resources building the world with all the neighborhoods and the lots, they could maybe make world packs as the OP suggested but it will probably take time, time that they might not have.
Those are my guesses anyway. If you ask me I think they should look into increasing their pool of resources by increasing their budget and raising their minimum system requirements a bit and see if that could allow them to make larger worlds. But I'm not sure if that's something that they want to do.
