While on Netflix’s Behind the Scenes podcast, The Witcher showrunner Lauren S. Hissrich explained the decision to adopt the original source material instead of CDProjekt Red’s adaptation.
The answer is very simple though and it’s that the video games are based on the books. They are an adaptation of the books. For me, it didn’t make sense to make an adaptation of an adaptation of something. What that does is allow us to tell The Witcher story but just in a different way. I think a lot of video game fans are concerned that we’re taking something from them. That we’re usurping something that’s theirs. And the truth is, I love the video games. They are not going anywhere. They are insanely successful. I can only hope that our show is that successful. The video games aren’t going anywhere and the books aren’t going anywhere. This is just a third way to enjoy these same characters and this same world.