But this begs the question: why didn’t they just write an original series instead of seemingly doing everything “wrong”?

You answered your own question, friend:

Sounds like Netflix writers are a little too eager to make this work their own.

They basically character assassinated and/or completely changed everyone, including the female characters they were trying to prop up as being strong (Yennefer in the Netflix show is a screaming whining mess who can’t do anything on her own and dicks over Ciri while Yennefer in the books and games takes shit from nobody, especially when it comes to Ciri and Geralt). They even (surprised I am even saying this) blackwash Fringilla in the Netflix show, whose entire point in the books is that she looks and is extremely similar to Yennefer during a time when Geralt and Yennefer were broken up. Them expanding on Istredd (except for the weird pedophilia with a young Yennefer) and the head teacher of Hogwarts Aretuza was pretty cool, but the writers literally shoehorned their own issues and takes on the show with no respect to the original material.

Fwiw, they also bullied the hell out of Cavill on set for trying to keep things faithful to the books and playing the games because A) he’s a Witcher fan and 2) it was helping him get into character.