Maybe there is some kind of negative reinforcement, too. The way it feels to me is: "I won't write documentation, because users don't read it -> Oh great, I will use ChatGPT to support my users -> The users don't know how to read documentation (or anything other than a 500-letters answer to a chat question) because they do everything with ChatGPT-like prompts -> I won't write documentation, because users don't read it".
Same for many things: most people tend to complain about CMake, but almost every time, I quickly realize that they don't know how to use CMake. Either they complain about it and never used it, or they complain and use it wrong. Therefore many devs try to build alternatives that will solve that problem by "being better than CMake". Turns out I have seen Meson projects that were a big mess, it's not just a CMake thing. On the other hand, people who learn how to use Autotool/CMake/Meson usually manage to make it quite maintainable.
Here the negative reinforcement would be: "People don't learn how to use the tools properly -> They make a mess -> They complain about the tool -> Someone makes a "better tool" -> People don't learn the new tool and make a mess -> repeat.
I truly believe that we could solve many problems by teaching people how to learn, instead of building technology that helps them being productive without learning.
Regarding the CMake documentation, though, I have been relying on it forever (that's pretty much how I learned CMake), and I personally find it pretty good. That is where I believe there may be some kind of cultural change: I am fine reading the manual (`$ man <command>`) and RFCs and documentation like the CMake one. It feels like "the modern way" is to find an example that does what you need and mimic it (hence the success of stuff like Copilot).
I just feel like my way is more powerful: I can both leverage official documentation (and RFCs, etc) and examples, and I can actually use them together (e.g. read the example, see new stuff there, and go read the official docs about those new things). IMHO, only being able to rely on examples that do almost exactly what one needs is a bit of a loss; people should learn to read actual manuals.
I read all the Unix man page of every command line program I used but there were only a few of them. There are a zillion of tools now, I can't spend days reading their docs: I won't be able to deliver to my customers and get paid. Googling how to do X magically works well enough.
Discord is the very last resort, only if desperate. I tend to keep very far away from tools that have discord as their primary documentation. There are always alternatives.
Trust is something that documentation for a project has to earn over time.
People will assume the documentation is incomplete and out-of-date, because that's the default for most things.
If it's comprehensive and stays up-to-date with new changes, the project community will eventually come to realize they and start trusting it. But it takes time to build up that track record!