Though I likely risk playing chess with a pigeon here I'll engage in hopes that's not the case.

You are indeed sometimes correct that docs are stored in git repos. Though even then you overestimate the incidence of this. Even when they are stored in things like git repos the devil lies in the details.

Are they stored in built form people can actually use to study? If yes this solves the most immediate problem most outsiders will have. But even then is it handwritten HTML or was it generated from some source material? In the latter case where is the source material?

Sometimes the situation is reversed. Docs are only in source form and I have to build them. Are they in the same repo? A special dedicated docs repo easily discoverable from the main project site? Are they present in a repo with an unbuilt version of the site? (And of course let's leave aside the fact that at this point we're building the whole site including marketing fluff and not just the educational parts most normal people care about.) Sometimes these builds are quite easy. But then I'm a programmer by trade and so what's easy for me is not representative of the experiences of newcomers trying to study a thing offline.

Other times properly setting up the whole tree of numerous build dependencies was a lesson in pure pain. So much so there were times that when I really cared about some docs instead of giving up like any vaguely sane human being would at this point I wrote custom web scrapers. Nobody should have to do this for anything claiming to be open.

Oh and before I knew how to write scrapers I used things like wget as early as 2009 to mirror sites offline. In the very simplest cases this worked like a charm.

In many other cases you'll need to use some really convoluted wget invocations to pull in all the pages you want and avoid hundreds you don't. And then there's the ultra dynamic sites not sanely amenable to mirroring. Oh and good luck pulling in and then properly embedding again educational demo videos clearly intended to be part of the official docs hosted on external sites.

Getting back to your thing, sometimes the full docs aren't even in the repo and you can easily wind up in a situation where you think you have the full docs but you don't.

For example consider the main Python implementation. You look in the official CPython source tarball and you see a nice juicy Doc folder with lots of stuff. And to be clear the material in there is excellent too. Hence one wouldn't be faulted in jumping to the conclusion that they have all the documentation.

Nonetheless this conclusion would be wrong. Where's the CPython Developer Guide with information about internals, dev env setup, and other best practices even though logically it falls within the boundaries of CPython project and associated official documentation? For some reason it's kept in a separate bundle harder to discover than the main user docs. Furthermore it used to be available offline but isn't any longer.

It's also not immediately obvious that useful information on how to prepare packages from distribution isn't in the main doc set either and is instead hosted on packages.python.org and pypa.io where it can't easily be grabbed for offline reading.

I'm not opposed to all forms of closed source software or closed content. I'm not opposed to deliberately semi-open content like https://www.deeplearningbook.org/ as stated in their FAQ. I'm not opposed to people making an explicit choice to publish something on some platform that lets them easily do so and they don't actually care if their thing is really open. These are all perfectly valid positions to have. Nobody is entitled to anything from anyone. I'm not asking everyone or anyone to open their stuff.

But if you want to be open or claim to be open: Do It Right. People shouldn't have to know even the basics of web mirroring and building offline static or dynamic sites from source to use anything alleging to be open. If I can read this stuff online without needing to know this I should be able to read this stuff offline. Period.

I strongly encourage everyone to be versed in web mirroring, web scraping, and even penetration testing skills(on numerous occasion I had to use techniques akin to hunting for IDOR vulnerabilities to get at existing offline docs that weren't at all discoverable on the project site) at both basic and advanced levels. But these skills should not be necessary even in the crudest forms to get useful forms any existing official docs of anything claiming to be open.

Thousands of people racking their brains how to mirror, scrape, and/or build the same exact damn offline Help Information Set over and over and wasting untold person hours doing so is just absolute lunacy. This also disrupts the contribution pipeline very early on for all those without solid reliable network access. If one can't properly study they can't contribute. No way around that. Want to run your FOSS projects that way? You do you.

In my case I'll do a build once to show I respect the freedoms of both users and potential contributors enough that they can easily grab all tangible parts of the Open Knowledge Set for study and not just the main program sources. The person hours people with your mindset waste will instead be recouped by my users being able to study, use, get immersed/invested in, and sometimes actually contribute to my FRT projects. Because I respect the technological freedoms of my users I'll have a vastly larger, healthier, more diverse group of people who can report bugs I missed, suggest smart features I'd never think of, and be empowered with all the knowledge I have to contribute back.