When you buy a TTRPG, essentially, you're getting a bundle of two (or perhaps three) things.
I'm going to put aside the third one, for now, and just talk about what I think are the big two in terms of page count: System Mechanics, and Game World.
Lots of people who are keen on system mechanics seem to feel that game world can pretty much take care of itself.
As long as the game master has a rough idea of what they want to present ("Hey, generic fantasy medieval! D&D-ish. Warcraftish. Lord of the Ringsy. Sorta like The Witcher maybe. Man, you know what I mean!") then the main bulk of the game book is preoccupied with what dice to roll in what circumstances, refering to what game system. To make things simple, adventures are often geographically isolated—an underground complex, a mountain pass, an unexplored island, an abandoned fort. Aaaand with that, we're ready to roll!
In the opposite corner, there are folks who feel that background is vital. They're happy enough with a rules-light system, just as long as the game world adheres to a particular canon. That means EITHER that the game refers to other media—films or books or comics—that the GM really should be familiar with before they get started, OR the bulk of the game book will be dedicated to conveying game world lore.
Okay—so the proposition of many rules light systems is, "rulings not rules"—in other words, the GM can develop detailed or specific rules systems on the fly, to cope appropriately with the particular path the players take.
And the proposition of many open world, or "sandboxy" systems is that the GM can develop the game world as appropriate, often according to complex tables (AD&D wilderness amd random encounter tables, I'm looking at you!) responding both to what the player charaters do, and to the development of the players' skills and objectives.
I suggest that it's quite possible to have a great deal of RPG fun in either circumstance —rulings not rules, or exquisitely defined system mechanics… emergent game world, or fabulously detailed canon. Or anything in between.
Right then. So, if we buy that, we've just established that NEITHER system mechanics NOR game world is vital to a great game. In fact, games can do without either… so can they do without BOTH?
The very existence of one-page, ultra-light RPGs suggests the answer is yes!
Okay, so if that's the case, then why do we sometimes have disappointing game sessions? We can deduce that it must be the case that the disappointment of a poor game session is simply not addressed by EITHER the game world OR the system mechanics.
So what does address the problems that result in poor game sessions? And why isn't that the main focus of game books?
I totally agree—but why don't players like an adventure? It only maybe because the adventure is poorly designed. (In fact, back in the days of Judges Guild I and my friends ran a whole bunch of incredibly poorly designed adventures, but still had loads of fun!) It could be about poor group dynamics. It could be about a disconnect between expectations and actuality. It could be about poor GMing techniques. Perhaps it could be about something else.
If the aim of a game designer is the help GMs and players to have the best possible experiences, then it surprises me that game designers don't attend more to the processes of preparing for and playing the game, of recruiting and selecting the right players, and giving them the right expectations.
Then again, maybe there are other factors that motivate game designers. Or perhaps people buy games for reasons other than to have a great play experience. (Both of these things are at least a little bit true!)