Players got used to the canard that things like unlocks and RPG mechanics were always merely ways of making multiplayer games more fun by giving players more options for how to play them. They got sold on the ideas that everything couldn't be available up front because, first, it would be more enjoyable to have slow progression because it would allow for even more ways of playing the game (it becomes necessary to try and find intermediate builds to use before you unlock, say, gun 3, which ideally is not just a straight upgrade), and, second, it makes it either less likely or makes it take longer for the community to settle into the ruts that build systems pretty much inevitably develop. Then, of course, people monetized progress through microtransactions (buy gun 3 now! buy faster progress! buy the newly released gun 4 so that you have even MORE option). Also, since these are games made by real people with motivations like greed and schedules, builds and progress ended up not conforming to ideals like actually offering players more choices in how to play. If gun 4 is a straight upgrade to gun 3, either for a given playstyle or just overall, then progression for that gun is just so much "wasted" playtime until you get the real gun. It's similar to how in MMOs there's the idea that the "real" content of the game happens after you've ground through 80 or whatever number of levels. In both situations, it's tough for players to just enjoy the ride because they're constantly telling themselves "the game will be more fun when I get (situation X)" which in many cases ends up twisting into "the game isn't fun before I get (situation X)." That fairly naturally led to people being willing to buy their way past the "boring" part of the game, eiteer with in-game currencies or powerleveling or achievement servers, and then with actual money when those methods weren't available or weren't fast enough.
Now, certainly some of that "fast enough" is due to gamer attention spans getting shorter, but at least some of it is also due to the gaming demographic changing. Young gamers grew up with these kinds of shitty practices and so they never got to experience fun that didn't involve grinding or unlocking things. Non-gamers entered the hobby through mobile games--the exact strategy that game publishers relied on to draw in people who'd be willing to put a buck in the mobile game's slot to keep playing it. So the composition of the gaming community changed: newfags with no sense were born or were led to become "gamers;" many old time gamers who knew that they were supporting shitty practices slowly cucked out and started playing games with unlocks and battle passes, which helped give the numerical support to devs and publishers who did those things to spin up the narrative that all gamers were okay with them. Even if a person didn't buy a battle pass or didn't buy in-game currencies, just playing the game let shitheads show numbers to other shitheads that led them to decide that their methods were sound enough to make money. Seeing those successes--really a lack of effective resistance to them--other big publishers started to push their devs to do the same kinds of things.
There's probably a really interesting history to be written about the rise of modern shitty practices like "battle passes" that might never get released, the "patreon treadmill" of giving money to devs or publishers for games that may never actually be complete the way they were originally pitched to purchasers, DLC and horse armor and paid mods, the tendency for publishers and devs to own the responsibility of hosting multiplayer servers (which stifled to death the modding/total conversion community and allowed them the centralized control to push ads and other shitty practices), and the expectation that games don't need to be finished and working and even reasonably complete when they're released (and the fact that the definition of "finished" is so nebulous today). Putting such a timeline together honestly would probably be the first step in educating people who play games as to why games are deliberately designed to be shitty these days.