People are so toxic because of the way the game works. Especially in lower elos, below plat I mean, people just don't understand basic macro and don't understand snowball system of league. As if for instance you have Jax who plays vs Fiora and fiora gets 1 or 2 kills. In lower elos instead of giving up farm and keeping the tower alive this Jax will just feed harder and harder and lead to a situation in which you play 4 vs 6 because Jax is 3 levels behind Fiora. This is the most apparent reason why people especially in lower elo flame so hard because other players might even unconsciously ruin their game singlehandedly. I know it's a team game but it's not a game like R6 Siege or Counter Strike where rounds last for 3 to 4 minutes and every round you more or less have equal chances of winning. In league the game usually snowballs out of control in addition to existence of super hard carry champions like Yasuo, Master Yi or Kassadin which are best picks for smurfs because are the easiest to 1 vs 9 them the game.
Another very triggering thing is the amount of AFKs in low elo. When I was in Gold 2 I remember that I had a match where I was playing Fiora vs Gnar and my jungle and mid went simultaneously afk in 4 min of the game. We lost in 21 mins and I lost -25 lp because of that and even though everybody reported them, nothing happened they didn't get banned or anything. They received the same penalty as the people that actually played the game 3 vs 5 and lost.
I get it that Riot can't just be permissive of not giving a penalty in lp when you lose a game but I personally believe that there is a huge amount of players who are just really good at the game and have their skill much higher than their elo suggests but because of the certain issues that Riot ranked system has they are hardstuck and as the time goes they become really frustrated and start flaming and may become oversensitive to other players feeding, not smiting well or just over that they play vs smurfs.
Not to mention the fact that even though you might go 0/10 and be completely useless in a match you may still be promoted to higher tier because of luck that your team is carrying you or just that you duo with a Grandmaster tier friend who boosts you while playing in duo.
Another thing are winning streaks that can get you in 30 matches from Silver to Diamond. This is a big bullshit because nobody is that good that they win every match even though they might just crush their opponents in lane or gank extremely well cause how you gank when your 3 lanes are 0/5.
It's nearly impossible to make a perfect match making and ranked system but there are so many evident error in Riot's that it's no surprise that people that want to climb as the time goes on become tilted and start flaming, trolling, afking etc.
In my opinion competitive scene in LoL is really healthy and developing but the balance in the game and just the attitude of people makes the game in the longer run unenjoyable, stressful and tilting. Especially if your favourite champion doesn't get skin in 4 years and Yasuo, Zed and Lux get one every month or when you main Aurelion Sol that just gets destroyed by the rework comparing to newly reworked diana which is completely broken and in high elo has permabanned status.