I don't think that is the entire truth there. Most games have a decent community, sure every community has it fair share of toxic players, but League is far worse than most games. What you are saying is that all games are toxic like league but that is probably coming from conformation bias while looking at other games known for having quite toxic communities, for example CS:GO. But not even cs is as toxic as league.
I left league 2 years ago because of a few reasons, i got bored, the community was shit and i started playing other games. I had over 1000 hours on league, i have over 1000 hours on CS, I play a lot of Rainbow six siege and overwatch. All are very competetive games, and from my perspective it is easy to rank the games communities by toxicity.
Starting with the least toxic, Overwatch most people in overwatch are friendly and dont flame, sure some do but it is easy to calm them down. After overwatch there is R6siege which is very close to how the cs community is by not as hatefull. the second most toxic game is ofcourse CS:GO, people here are very competetive and flame bad players quite often, but it is not all that rare to end up in a team where people are just having fun. And the game that I've found to be the most toxic is league, and this is by a large margin, in league you get flamed instantly when you get ingame if the other people don't agree with what you picked or what you were thinking of doing that game, when someone joins and says that they havent played in 2 years, instead of getting tips on what to do, and what has changed the get flamed for buying bad items and playing "wrong", when the items and playstyle used to be good.
Excuse my bad spelling and grammar, I didn't have much time to write this and i don't really care enough to rewrite it.