Three things have changed.

First, GameFaqs put all of it online for free. Why would you buy a book? It's now a relic of the old web, but it's still there, filled with cheat codes and guides for all your games. Strategies evolve as players learn new things, so forums have replaced prescriptive guidebooks to accommodate new ideas.

Second, game development has changed. Cheat codes were originally tools for developers to function test. To test a particular level or feature, devs would have to play the actual game. Modern games are not as linear, and modern developers can throw together a test environment on the fly. Game components are more like isolated microservices, so modifying the game to test features does not need to be baked into the code.

The third thing is that everything has an online multiplayer now. Cheats are fun when it's just you against the machine, but online competitive play is ruined when your opponent has infinite health. Online cheaters still exist, ruining multiplayer for entire communities, but their aimbots and shit cannot be officially sanctioned or promoted in a guidebook.