I think for the most part, modding is probably an afterthought, if it's thought of at all. Since they are likely short on time already and then modding is kind of a gray area anyway.
That said, there are a lot of things where it's possible, but few know how and then there are some key things where it's kind of beyond the design of the game.
Like the holiday icons list uses a simdata file with references to the icons, which means you can inject to the xml list of holiday icons to avoid conflicts, but it won't do anything because you still need to add your new references into the simdata, which is handled on a level where you can't just inject to it. So if anyone wants to add new icons, it's gonna have to be one modder doing it and that's it. This is one kind of limitation.
Another is just the UI in general. Anything that uses a precisely-designed UI menu, such as the fame menu, is mostly beyond modding because it's done in the flash and there aren't really tools available for handling the GFX flash files well. And even with a tool to edit them correctly (which is a pain as it is) you'd have that usual problem of conflicts if more than one modder edits the same file. There are certain types of menus that can be done somewhat flexibly, like picker menus, but these have limited use.
So as you can probably guess, some of the issues of stuff not being moddable is because it fits in the context of limitations like these and so short of the sims team redesigning how those systems work, it's not gonna happen. I would love for certain things to be more moddable, but I'm doubtful they would see reason to prioritize such, unless they can somehow justify it as part of a new/modified feature of the game.
Then when it comes to the "few know how" problem, well that's mostly a problem of skill and time investment and all. We lost easily one of the most knowledgeable and helpful in losing Scumbumbo. A lot of modders learned things from him and had work made possible from tools of his. Which is another thing about it... even though this game supports modding to a certain extent, people have had to make their own tools in a lot of cases to do modding for it. Modding always has a skill problem because so much is hidden or hard to get at, and we have no access to things like in-house tools. In an ideal world, the skill required would be a lot lower, but as it is, the closest we get to that is through community-made tools, such as Zerbu's Mod Constructor, that are designed to cut down on the technical knowledge you need to make stuff.
