The premise of long-running TV show Dr. Who is that 'The Doctor,' a powerful alien, travels through time and space with a human companion. This setting is great for TV because time/space travel means that each episode can have a different setting to keep things fresh. The human companion provides a reference point for the audience; if the Doctor was working alone, he wouldn't be constantly explaining things to a 21st century human audience (this may also be the reason why the main character in The Last Samurai is an American).
The setting of The Witcher is good for a role-playing video game. The player controls a superhuman monster hunter, which allows for:
1. Regular travel and combat
2. A conventional quest structure
3. The player character being good at everything
4. The player character being treated as an outsider by non-player characters
Point #4 might be most important: in almost every RPG, the player doesn't 'belong' to the world they're acting in. The player is most often an outside force, swooping in, solving problems, and making things happen. Some of the most exciting moments in RPGs are reacting to the plot-influencing choices of non-player characters.