Most ttrpg books sell to GMs, who only make up a certain percentage of the total hobbyists.
I honestly wonder about that.
I'm sure that it's true that a large majority of ttrpg books sell to a small minority of ttrpg customers, but i suspect it's not as simple as the GM/player divide. The hobby has a lot of completionists, a lot of FOMO, and a lot of collectors, and also a disproportionate representation in fields like IT which tend to be high-paid and who can affort to lash out on expensive hobby stuff as a self-indulgence. I suspect with (a near-complete lack of data to support my position...) that a significant proportion of RPG books that get bought (especially non-core or third-party books) get read but probably don't get used in-game.
Example - me. I started buying D&D in the late 2e era. My first actual game was early 3e, which was followed by running a d20 Star Wars game and then a 3.5e game, then a long hiatus from D&D until i joined a 5e campaign (a relatively vanilla Storm King's Thunder campaign where the DM allows material from a limited array of books). There were also various abortive or short-lived forays into playing Champions, V:tM, Mutants and Masterminds, and a couple of others, none of which games probably lasted longer than a couple of months.
Despite that, I own a vast amount of RPG books. Near-complete sets of the various FFG WH40K games, most of Exalted (all of the editions) the core books of basically all the WoD books (and most of the range for Vampire), the entire Mutants and Masterminds range plus a fair whack of third-party material. All of 7th Sea 2nd ed, Alien, Call of Cthulhu, Blue Rose, Ars Magica, Pendragon, RIFTS, Deadlands, Blue Planet, Lace and Steel, and shelves upon shelves of 2nd and 3rd edition D&D material, and that's before i even get into 5th ed or digital products or the boxes and boxes of miniatures i bought back in the 3rd ed days.
Buying RPGs is sometimes a completely separate hobby to playing RPGs, is what I'm saying.