why can't GM as referee just follow the rules? Why must they be counted among players?
GMs are and always have been Players. Thats true whether you look at the game as elaborate Improv, a board game, or the nebulous non-game-thats-better than those icky board and video games that some people want to believe in.
The rub comes from the fact that GMs have a significantly different role from a colloquial Player, serving in some capacity as the underlying engine for the gameworld. Whether that means simulating a gameworld or just filling in whenever NPCs have to do something is immaterial; GMs facilitate there being a gameworld through rules that enable them to do so.
Even FKR works the same way in this respect, and the only games that defy this structure are solo and co-op games, that shift and distribute that role across the game's procedures and/or other players.
Id argue that historically, the single biggest reason people break from the rules is just rooted in bad game design. I know people typically don't appreciate me passing judgement on their favorite games, but fact of the matter is, people don't go out of their way to break the rules of a game they enjoy unless they have a neurotic compulsion to cheat, which isn't something we can solve with game rules.
Ergo, if so many are refusing to follow the rules, it can be argued the reason lies in the game itself.
Anecdotally, I can point to my favorite two games, as examples. DCC and Ironsworn are games that, relatively speaking, didn't induce any desire in me to change anything about them compared to something like 5e, which I modded to hell and back, or something like Masks, which I so firmly rejected I didn't even bother trying to address it.
Why those games work for me in that way is that they're simply inherently fun in a way pretty much everything else isn't, and while surely a lot of that is rooted in a matter of taste (I clearly enjoy the gonzo nature of DCC, and Ironsworns focus on solo allows me to grok its PBTA-esque design in a way others in its heritage don't), a lot of it is rooted in how the games work mechanically and the overall gamefeel that both produce.
DCC in play is everything DND wants to be and theres seldom a dull moment as other players turns are actually fascinating to watch,
especially if they're magic users. Ironsworn, unlike most of the PBTA heritage, is practically always in a first person perspective and this produces a very satisfying aesthetic that works well with its mechanics, making it feel less like I'm following an obligate path and more like I'm playing in a real sandbox, which is my ultimate preference.
Design wise, I personally think its a waste of effort to worry much about people breaking or otherwise ignoring the rules, as what we're really talking about is the game coming into contact with players, and even sans any rule breaking or ignoring, the game is likely going to be played differently from what was expected or intended, and as a designer you have to be willing to accept that that is going to happen.
Video game people learned this decades ago, and this is the underlying logic of design methodologies that focus on chasing the fun. In an rpg, if it can be observed that enough people are consistently ignoring a rule, the simplest solution isn't to try and force the matter but to simply remove that rule.
But, if the rule is important, then we can start examining why it isn't working and what sort of issues is it causing in the game feel.
This methodology was actually the entire point of
how I approached Crafting and Degradation in my game.
It was desirable given my game's overall theme that Crafting be elaborate and powerful, and that lead to the design solutions that has Craftables sharing ~50% of the load in terms of combat balance, and in turn necessitated a means of encouraging crafting to be a continual part of gameplay, rather than a one-off activity you do as early as possible.
Hence, Degradation mechanics, that I designed to specifically counteract the most common complaints other games have produced with such mechanics. You can use your items at 100% effectiveness for at least some amount of time, and losing an item permanently is always a choice rather than an obligation. You're simultaneously encouraged to stick with specific items over time, but also to eventually move on to new ones, and its a choice which way you go.
Now, no doubt, if my game takes off, there won't be a shortage of people who would still try to ignore these rules. But that can be understood to just come with the territory, so there's little reason to try and fight it. And that is why I'm very, very careful about not designing the game with breaking points; rules that, if ignored, make the game completely break down and be unfun. Thats why my game is a tactics game for combat; you can't cheat your way around player skill being necessary, regardless of whether you made the party OP by letting them ignore some of the balancing mechanics.
Ideally, the only wrong way to play a game should be to just not play it at all.