It makes some sense that the story writter of a story-heavy game would never play the game as a game, for fun, same as a book author not reading his or her own books for the pleasure of it.

Or to put things another way, they've already "consumed" the story in the game before ever playing the game.

As it so happens I'm making my own game, which is not story heavy, so I hope I'll actually be able to enjoy it myself, but I can see how at the end of making a game you've just seen too much of it in too much detail from too many angles to actually be able to enjoy it yourself as just a game unless, maybe, a lot of it is somewhat unpredictable even for the author (i.e. stuff with lots of procedural generation or where what happens comes from the complex interaction of various game mechanics and user choices, which is what I'm aiming for in my own game)