Games like Knock Knock!
Games like Knock Knock! include Firewatch, Layers of Fear (2016), Poppy Playtime, What Remains of Edith Finch, and Outlast. We matched them by the tags players give Knock Knock! on Steam (Walking Simulator, First-Person, Noir, Dark Humor, and Horror): closest matches first, and among equally close ones the better-known games.

Firewatch
Shared: Walking Simulator · First-Person · Adventure

Layers of Fear (2016)
Shared: Walking Simulator · Horror · First-Person

Poppy Playtime
Shared: Horror · First-Person · Singleplayer

What Remains of Edith Finch
Shared: Walking Simulator · First-Person · Singleplayer

Outlast
Shared: First-Person · Horror · Singleplayer

Dark Deception
Shared: Horror · First-Person · Singleplayer

The Stanley Parable
Shared: Walking Simulator · First-Person · Indie

VERGE:Lost chapter
Shared: Walking Simulator · Horror · Indie

Blair Witch
Shared: Horror · First-Person · Adventure

The Beast Inside
Shared: First-Person · Horror · Adventure

Slender: The Arrival
Shared: First-Person · Horror · Indie

Among the Sleep - Enhanced Edition
Shared: Horror · First-Person · Walking Simulator

Five Nights at Freddy's: Security Breach
Shared: Horror · First-Person · Singleplayer

The Beginner's Guide
Shared: Walking Simulator · First-Person · Indie

Portal 2
Shared: First-Person · Singleplayer · Dark Humor

Bendy and the Dark Revival
Shared: First-Person · Horror · Singleplayer

The Hat Man: Shadow Ward
Shared: First-Person · Horror · Indie

Amnesia: A Machine For Pigs
Shared: Walking Simulator · Horror · First-Person

Still Wakes the Deep
Shared: Horror · First-Person · Walking Simulator

Bendy and the Ink Machine
Shared: First-Person · Horror · Singleplayer

Garten of Banban
Shared: First-Person · Horror · Singleplayer

Spooky's Jump Scare Mansion: HD Renovation
Shared: First-Person · Horror · Indie

Visage
Shared: Horror · First-Person · Singleplayer

Scanner Sombre
Shared: Walking Simulator · Horror · First-Person