These sims were designed so they
can be modded. THe reason is simple:
1. Modders have more time to tweak performance issues and set preferences for a wide and varying enthusiast base.
2. The devs couldn't spend their time on fine tuning every aspect of the sim. It's just too much to ask, especially considering release schedules.
3. The more moddable a game is, the more interest it generates from the gaming community. The more interest it gains, the more copies you sell. The better the modders do with a title, the easier it is to sell an expansion.
End result is everyone is happier
It's just plain good business to make a sim like this customizable.
And for the hardcode in the program? The devs don't make everything available. The guts are out of reach of casual modding.