anjohl: You don't read a book, and cross out words, or watch home movies on your phone during a film, so why tolerate brutishly inserted content in a game you consider a work of art?
People do write fanfictions, create crossovers, or even other famous authors write other books from the same universe to enlarge it (see Dune for example.)
Still, you are utterly wrong. As PetrusOctavianus pointed out, videogames are, by essence, a completely different form of art from books or movies. Any game can be programmed in any way creators desire: There are games which are completely locked out, and those make it clear that creators do not wish their creation to be meddled with. On the other hand there are games like The Elder Scrolls which embrace their community and its additions to them, and are indeed better for it. If I have to pick between a game which is completely static or a heavily modifiable one, I'll grab the modifiable one, because I know that if something bothers me about the experience, I'll most likely be able to modify it.
And if all the fan additions are 'brutishly inserted content', why is it that so many famous moders ended up being hired by the very companies whose games they have modified and why games, most noticeable in the Elder Scrolls series, pretty much add content from the most popular mods to next iterations of their games?
What you are judging so harshly is, in fact, a unique interaction between authors and content creators. Authors can't be perfect, certainly not in something as dynamic as videogames - their community, on the other hand, figures out fairly quickly what have they screwed up, and if it's given tools to fix it, they will. And they will, of course, add their own additions, for everyone to try - like Frostfall mod for Skyrim, for instance, which tracks body temperature of your character. Basically, you can freeze to death if you're not careful enough, and it goes so well with the world of Skyrim - it's just dumb and immersion-breaking to listen to everyone talking about chilling cold, yet swim in water filled with glaciers in your underwear.
And the best bit is: It's all completely optional! And that's precisely part where I just don't get you. Why would you try to condemn something you don't even have to use yourself? If you want to play games vanilla, you can, nobody is stopping you. Fact is that those content creators you value so much clearly wish for their work to be modified. Who are you to tell them otherwise?