One factor here is because they simulate maps to appear larger.
There are only 4 or 5 lots per area in Willow Creek; however, they stuck in a ton of fake housing, etc to simulate a bigger world.
Our 4 or 5 lots can be surrounded by 30 or so fake houses, etc.
Foundry Cove - Willow Creek:
https://vignette.wikia.nocookie.net/sims/images/6/6e/Foundry_Cove_from_above.png/revision/latest?cb=20141230152650
So, the problem is there is no such thing as an empty world in TS4. Newcrest is full of emply lots, yet when you go there, it has tons and tons and tons of fake housing and shops, etc.
EDIT:
My point being that is not easy to just make these things - ie just hand over an empty world - full of empty lots - because of all of the fake scenary that they have to include.