Of course you're right that video games should have interactive stories, that are influenced by the decisions you make while playing it, which is an entirely different thing from a plot in a book. But surely it becomes easier to make a good story for a video game if you've got the world-building and three-dimensional characters already fleshed out for you, and they're already good. Okay, it also has its downsides as you may not be as free in the twists you can create as you would be if you'd started from zero, but I would think the advantages outweigh the downsides.

I haven't played the Witcher, nor read the books, but I do have a basic idea of what the game and the books are about, and it is my understanding that the books and the game both are notable for their three-dimensional characters and refusal to buy into simple concepts of good vs. evil. And those, plus of course the general plot lines of the books, and perhaps some descriptions or pieces of dialogue here and there, seem like a lot of useful elements for game writers to work with. They still have to write a good game plot themselves, but seems to me it should be easier, and one should be allowed to have higher expectations, when the game writers are given all of those elements beforehand, allowing them to focus their creativity purely on how to turn those elements into an engaging interactive game plot. In the case of extremely famous books like LotR, I admit that this might actually hamper the game writers' creativity as they don't dare to deviate from the plot too much, but Sapkowski's books are hardly that famous that the game writers need to worry about deviating from the plot when that's necessary to make an engaging game.