Wtf are you talking about?
How many movies/tv shows have been made based off a book that followed it to the T?
Closest book adaptation movie ever made was The Godfather 1 and 2, even then there were a few changes, and many parts omitted - but the essence of every scene in that film fell off the book pages.
Almost every other adaptation from book to film have remade their version of the book to fit the medium of television and/or film.
The fuck, you watched Game of Thrones and you thought to yourself "this is a very faithful adaptation of the book?" - because, now here's a surprise, it wasn't. Many things were changed.
You're opinion is faulty on many levels, and you should
just come out and say you disagree with the casting instead of making a baseless assumption of what and how a creative vision is applied in the adaptation from any medium into film.