The lack of thematic tension and thematic horror stems from the fact that Knock Knock presents itself as an intellectual puzzle instead of as an emotional story.
Knock Knock purports to be game about hide-and-seek. The player controls The Lodger, a solitary man haunted by insomnia and ghosts. Every night he wakes up several hours before dawn, and you must guide him through his house, turning on lights to make sure everything is still in order while also turning off lights to stay hidden from malevolent spirits. If you find a Lodger-shaped clock, you can speed up time, bringing you closer to the safety of dawn. If a ghost finds you however, the time is set back several hours, extending the night and the game.
Those are the mechanics, in brief. The story, or even any semblance of a story, is purposefully vague at first, but over time, specific mysteries start to take center stage: Who is this lonely Lodger in a cabin in the woods? Who or what are the ghosts? Are they real or imagined? Where did The Lodger’s diary go? Is it even his diary? What can’t he remember? And why/how did he forget it? Who is the little ghost girl? Why are the spirits haunting/hunting him? What is going on here?