The percentage of players who actually read game manuals is a small minority to begin with.
I am playing devil's advocate here because I've been gaming for over 30 years and when I was younger and I actually bothered to read them and appreciated it when game publishers put some effort into them, they were a part of the overall gaming package every time I bought a game.
But, cost savings aside (which is not nothing) and materials aside (ultimately, most of them end up in a landfill if one doesn't recycle them), most people simply don't use them. As it's been pointed out, it's too easy to do an in-game manual and that's if a game doesn't teach the player the controls as a part of the game.
Over the years, game manuals in general (particularly over the last generation) have become little more than a layout of the controller button map with a bunch of warnings, warranty info, legal info, etc. taking up the majority of the insert. I don't care if that's in there.
Granted, if a publisher makes the decision to create a fun manual with a nice layout, color press, production art, etc. that's great; it's something even if most will only flip through it if they even bother with that, but game manuals like this were pretty rare even before it became the trend to stop including them at all.
The reality is unless one peddles in trading and used games, they're all ending up being recycled or in the landfill anyway and I know not everyone who claims they're important or how much they like/want them won't be keeping all their games infinitum. I've kept most of mine over the decades, but I'm an extreme minority.
I don't have a problem with absent manuals and if an old guy who actually read them and collects them can deal with it, I find it hilarious that anyone much younger than me is having trouble with that.