I think the whole thing just comes down to "game design".
They either let you roleplay
a character within the game context or force you to roleplay
their character.
The Witcher games come from a quite popular serie of books, an its target audience are the people that read those books and want to play as "Geralt". Allowing a "Geralt" that strongly deviates from the one in the books will most surely upset the fans, and would make the whole point of playing as "Geralt" pointless.
Then we have PS:T, as far as I know, there's no books about Nameless One that could influence the players perception of that character so it is easier to allow a broader range of personality options. Also the game itself is about the discovery of
who is the Nameless One, what led him to become inmortal, and the consequences such a thing brought to the planes, and as you discover this you sort of have to decide what to do about the mess you left behind, so the game acomodates for this.
So, as I see it the voice should only be the icing on a cake instead of the dough for the cake.
I hope I made some sense