This fascinated me ~90s computer gaming. There were clear camps of "people who always read the manual" and "people who never never read the manual."
I think I was the only one who really cared about it. I've had it for months, I think. Read it cover to cover more times than I could count. Pretty much memorized every bit of lore and trivia there was in it - and there was quite a lot. The manual itself was a small booklet, with a couple pages of backstory for each of the playable species, plus a lot of extra world-building details about individual units, buildings and planets. So even as I didn't have access to the game, my own imagination built a whole world and infinite stories out of what some would consider "filler content". But this filler content made me care, and eventually shell out my pocket money for Brood War, and later on, money from my first full-time job on the collector's edition of StarCraft II.
That SC manual came probably at the tail end of the era where the publishers put any kind of effort into manuals. Case in point: SC:BW came with just a tiny booklet with about equal split between lore/context, instructions for the new and changed game aspects, and legal/credits. SC II came... well, I don't know with, by that time Blizzard stopped caring about the game story anyway.
(Oh, and speaking of warm memories of computer manuals in the 90s, I have a special place in my heart for the big, heavy, couple hundred pages long manual to Word 6.0 (not 100% sure about the exact version), and earlier, for the interactive self-training application that came included with Windows 3.11 - that one taught me how to use the mouse!)
More interesting in retrospect, the first manual contained extensive descriptions of kiith clans. They go on for dozens of pages, and don't show up in the game at all. Someone spent serious effort writing them. Why? Left over from an earlier version of the game pitch where the player would have to manage relations between competing groups on the mothership? https://sierrachest.com/gfx/games/Homeworld/box/manual.pdf
Skimming from the dozens of pages I would love to have read back then, I think this may be the case of writers creating a detailed setting for the game early on - only fraction of it could become part of the gameplay, but presumably designers and writers were using it as a reference. And, come release time, it didn't hurt to include the world-building bits - it might just get some players to enjoy the game more (I know it would've had this effect on me), while the alternative, back then, was for those details to never see the light of day.
And/or maybe they hoped to make some follow-up games, or tie-in novels, etc. - then including those details was both important to solidify the fictional universe, and would drive some secondary sales for the game itself.
Some board games come with pretty hefty manuals, still. Twilight Imperium comes with a whole separate lore booklet that has nothing to do with playing the game.
Pretty interesting idea that eschewed technological tricks to prevent copying.
Which specific version do you distinctly remember?
Especially common are the people who get others to read the manual for them and answer all their questions, because they themselves obviously have no time to be reading them but more than enough time to grab some unfortunate sod to do so.
And about the instructions. Sure, it is useful to read them, but even more useful, for all humans, is to rely on the training. We don't carry around dictionaries to talk, we don't read "Introduction to Javascript" daily, we don't use UX designer guideline book to open a faucet in the bathroom. And re-training is not free, that's why so many people jumped in based on their old skill. Not based on their ignorance, but on the skill, and that's important distinction. People actually knew how to play such games, that's why they subconsciously omitted instructions.
Not in tradle, for example
This happens in my life constantly when dealing with family IT problems or mentoring juniors at work (and sometimes working with seniors too!). People just do not read the output of their computers.
If you can constantly guarantee that everything work the same all the time, then there are fewer issues. However, it does not, and because computers are prone to both bugs and marketing gimmicks, it is easy to burn out on a mental model of things.
People who do read the output of their computers experiencing situations where the computer doesn't do what it says it does, can quickly burn out. Not to mention, reading only what the computer says is a good way to diagnose the wrong thing. You observe something similar when a feature or bug is exposed and the first response is "well it used to work before" - which is often (partially) true, but that does nothing to explain why it doesn't work or did work.
Then there is social and linguistic familiarty (often iconographic). It's telling that the way the IQ test is most flawed is that it scores those with more cultural familiarity higher - things that seem "easy" may only be so because of a certain familiarity, with where things are commonly placed, common functionality, etc.
And again, as UX designers destroy these foundations in the hopes of making their UIs "easier", it only makes things more difficult, not less, to understand.
Unfortunately, that sucks even more from a UX design perspective, since it means even if you absolutely nail the instructions you need to know all of the surrounding context and find a way to address it.
From my experience, most people don't read the instructions or text notes anyway. So its possible that an even better UX is one where the user figures out what they can do through intuition rather than reading.