CSGO, overwatch, COD, apex legends, PUBG to name a few
Notice the difference: In those games, you have to have an entire team, or at least almost an entire team, who suck to lose the game; in League, due to how the genre itself is conceptualised, you only need 1 player to lose the game. (EDIT: It's not the best post opener, please read on)
As someone may have discussed in far more detail before, League's toxicity problem is in part (and by NO MEANS entirely) due to the fundamental design of the genre itself. The elevated frustration is kinda just what happens when you have the elements of roster, the existence of snowball and feeding, team focused gameplay where no character can do everything (until they started pumping out variants of Yasuo, Vayne etc that is), and the emphasis of drafting in which you can't change character after going into a match, all bundled together in the same experience. The factor is that players are more likely to feel the outcome is out of their control, hence the arguably higher frustration when things don't go there way.
Shooter games are usually either a bunch of faceless nobodies that all theoretically the same dude just with different guns which you can switch on the fly, or it does actually have rosters like Overwatch but you can still switch on the fly regardless so it's kinda the same deal. This may cause one to think and FEEL that you can remedy it even if someone on your team screws up; not so much in League, as previously discussed.
I wouldn't call the design of MOBA/ARTS being like this a design flaw per se. But it is indeed troubling that such combination of designs that are there to emphasise and force teamwork ultimately are responsible, or at least partly responsible, for player toxicity.
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Another factor is as previously discussed. Like the game is super accessible, especially given it's f2p and that it also runs on just about any toasters you can find, so it's sort of expected that you can find the most low-effort assholes. Or the lack of voice chats. Etc.