I don't disagree with the observation that a lot of folks in tabletop roleplaying spaces don't believe that game design is real (i.e., in the sense that they believe any GM should be able to achieve any experience of play using any system, and refuse to recognise that rules are opinionated about what sort of games they want to produce), but I feel like putting that at the forefront is confusing the symptom for the disease. A lot of folks in tabletop roleplaying spaces don't believe game design is real because they don't believe that games are real.
I've talked in the past about how Hasbro's efforts to deceptively market Dungeons & Dragons as universal entry-level game have fostered a culture of play in which any appearance that D&D isn't a universal entry-level game is regarded as evidence that you have a "bad GM", and how, in order to avoid being a "bad GM", it's necessary to treat it as a normal part of the GM's responsibilities to constantly monitor the outputs of the rules and quickly paper over any gaps between the game the rules want to produce and the game the group wants to play, like a cartoon train conductor frantically constructing the very tracks along which the train they're conducting is riding.
The trouble is that most players aren't stupid, and readily see through the act. They (correctly!) observe that the particulars of the rules don't actually seem to matter all that much, because most of the desired experience of play is the product of the GM's constant interventions, rather than the product of interpreting the outputs of the rules – but instead of identifying this as a problem, they conclude (again, quite reasonably, as they've probably never seen it done differently) that this is what tabletop roleplaying is. The GM merely pretends to be moderating a game; in truth, they're a pantomime-leader whose job is to maintain the illusion that we're playing a game with rules, when in fact what we're really doing is guided improv theatre.
And of course there's nothing wrong with guided improv theatre – it's a fine pastime, and one I've enjoyed myself on many occasions. However, it does put folks who really do want to play a game in a bind, because now there's this insurmountable communication barrier. You can say "I want to play a game, and these are the rules of that game", and receive what seems to be enthusiastic agreement with that premise; however, a significant portion of the people expressing that agreement think they're participating in a bit of kayfabe, like very dedicated professional wrestlers who stay in character even outside the ring.
Critically, nobody is necessarily acting in bad faith in this equation. The folks who don't bother to learn the rules because they think games aren't real mostly aren't fucking with you on purpose; they honestly thought they were yes-anding your improv prompt by pretending to care about the mechanics of play, and when they discover that you really do expect them to do all that fiddly dice math, from their perspective it genuinely looks like you were the one misleading them. It's just a fucked up culture of play garbling all the signals in both directions.
(Note that, while I've identified Hasbro's deceptive marketing as the ultimate source of this culture of play, indie RPGs are hardly innocent of perpetuating it. You only need cast a critical eye on the "Rule Zero" sections of many popular indie games to notice that their authors are all in on the idea that games aren't real!)
