Maybe because game companies went from being run by geeks to being run by business majors.

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.

Cheat codes were originally what they used to test the games. Now they have proper dev tools.

But gamers have mods now

Because why give you free cheats when they can sell you DLC?

Let's plays and playthroughs are the guides nowadays. I don't know about cheat codes.

You're posting on it.

I think these were mostly replaced by art books due to the info from guide books now being on the internet. I collect a lot of art books from games I like, though not every game puts one out.