First note.
PS:T did have a novel version published. Amazon has it
here.(That's the American one, not sure what the address is for the European.

Also if you're a die hard fan of
PS:T don't buy the book without at least using the Search Inside option. You almost certainly won't like it as much as the game.)
Second note, I've never played
PS:T, never read the novel. What info I have of it is from other players, mostly here(most seemed pleased to passionate to obsessed). So I can't completely comment on its skill in creating a literary masterpiece.
However I can make an important distinction. A game does not need to be as excellent as a literary work. Why? No not because gamers are unrefined and don't need as excellent a story. A game is a very different work than a piece of writing. A piece of writing requires you to work twice as hard to establish empathy with your characters, to create passion for the plot, anger for the villain, etc. With a game, you are the character, you spend time creating him, and then as the plot develops, more often than not, you learn more about him. Not just strengths and weaknesses, but you discover who he is, what he is, his past and you develop his future yourself.
In a game the protagonist is yours. In a book he belongs to someone else, he's distant(or she as the case may be). Thus for a game, you are more easily drawn in then a book. In the end, the story of
PS:T or
BG2 may not be as epic or poetic or whatever as some of the other stories you consider true works of fiction, but because of what they are they don't need to be to be close to us.
I'm going to shut up now. Didn't mean to write a novel here.
"She could resist temptation. Really she could. Sometimes. At least when it wasn't tempting." - Calli Slythistle
"She was a fire, and I had no doubt that she had already done her share of burning." - Lord Firael Algathrin
"Most assume that all the followers of Lathander are great morning people. They're very wrong." - Tanek of Cloakwood
we are all adults playing a fantasy together, - cmorgan