Virtual road rage is certainly a large part of this--I always use the analogy that if you have 1000 twitter followers, and they are wonderful people all the time but each one only says one nasty thing a year to you, if they all do it at the same week or whatever it's gonna be
brutal.
That said, I think there is also a lot of justified anger.
In a world that is increasingly terrible and hostile in a lot of ways it's really important to have a safe place, a place to unwind and settle down, place where people can tune in and drop out. Video games are really great for this purpose.
But a lot of devs don't seem to recognize that this is an important purpose of their product, either going low-brow capitalist (say, EA or Zynga or Bethesda) or high-brow artiste (Phil Fish, Jonathon Blow) and alienating their playerbase by breaking games in order to create higher profits, by patching games endlessly in the name of "gameplay" (Darkest Dungeon is a great example of this, ditto a lot of Blizzard's stuff), or by taking very public stands against various sacred cows of their customers--that is when they're not actively saying nasty things about their customers. As a gamer, why wouldn't you voice your displeasure at the people who are screwing up your ability to calm down and relax?
Similarly, even on the art angle, a lot of publishers and devs make compromises to push a message or sell a product. Like, can you imagine the backlash at a Holy Bible 3.0: HD Edition 2018, with DLC psalms? Game devs and publishers do this all the time. Imagine you heard they were making Rent 2.0, but everybody was straight and monogamous and only had the flu and were upper-middle class. Even if it was done well, even if it was a good story, it wouldn't be Rent. That's even assuming the change was done expertly--in video games it's usually a studio or a publisher chasing some market demographic or "industry trend" causing that violation of the IP and seldom is it even done well! So, if you're a gamer, why be nice to people that are willingly violating an IP that you love?
Further, on the PR front, gamers have gotten used to being fed thinly veiled bullshit as the industry has "matured". The same flaccid attempts to gloss over bugs or mistakes, the same lame attempts to use identity to sell a game, the same boring controversies and plant articles used to increase air time...it's insulting. So, why have empathy for (some random indie dev) when the industry as a whole is not behaving in good faith? Why not treat every dev communication as a weakly-spun PR gambit? Why show any charity at all?
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I agree that this is bad behavior, and I agree that drive-by virtual road rage accounts for a lot of it...but not all of it.