The book is basically the same heavily scripted thingy like you have in the game, relying on deus ex machinas all the fucking time, linear. Hell, the basic idea of tunnels and metros - they are all linear. I don't get why everything these days has to be some boring open world game. You play Artyoms story and he didn't explore the metro system for random things and instead had to do his mission.
The Vaultdweller had to find a waterchip and destroy the mutant threat, what was quite linear task. But i don't remember Fallout to be strictly linear.
A book on contrary to the game needs to be linear. So they simply made a book into a game, not the universe of it.
But the book also have a map of the Metro, which suggests, that the quest of Artyom as a part of something big. Something that inspired people to write their own Metro stories. It has abandoned, "mentally dangerous", irradiated, infested stations. Sure, Artyom hadn't need to explore the system to find some puzzle parts, like knowledge like mutant sterility in Fallout, artifacts, arcane technology like bunkers or cars or weapon systems, as he hadn't much time or motivation to do it - but this would be a great stuff for a RPG.
Cry more about open world, but this is a reason why you played RPGs and Fallout in particular.
