Manuals became victims of publisher cost-cutting, though publishers like to claim that it's "better for the environment" for optics reasons.
I used to always read manuals, cover to cover, when new games came with them. I liked reading about the characters, the backstory, and the little tips and tricks that were sprinkled in. When playing a game, I liked having the manual out in front of me to quickly check on play controls or how to pull off certain moves. I was always happy when publishers added maps or posters in the game package, too.
In-game manuals are a pain in the ass, and you can't read them unless your console is on. Internet resources are a better solution, though looking down at a small smartphone screen to try and decipher how to pull off certain moves can be a chore-- not to mention the extra time it takes to pull up the appropriate website and navigate to where you need to go.