Because I write software as part of my day job (I do research in academia, so I would not consider myself a "professional" in software development since I am more a jack of all trades, but I know enough about the process to tell you that it's not as simple as you think). I can't give you links from TW since they are not exactly forthcoming when it comes to the development process, but
this one I think will give you a good idea of the kind of problems a software developer can face when adding features.
The thing is they would not do the same thing that a modder does. Modders do not have to worry about how their code plays with the rest of the game, they don't have to think about how their contribution fits with the overall software architecture of the game, and they don't have to worry about maintainability (if I make this change to the code, how will this affect further changes? will I still be able to change the other thing if I do this?). The more complicated a piece of software is, the more careful you have to be when you make changes. And typically you try to divide the code in individual, separate units so that you can change one without affecting the other, but again when the project becomes complicated it's not always easy.
Note that I am talking about mods that make significant changes here. If you just go and makes minor changes to some xml files then yes, that's easy either way (but honestly, that's something that anyone could do).