"Why are people so toxic in this game?"

Well its probably because it starts when that person's match is ruined by another person who had their match ruined.... etc, etc, its a chain of griefing in essence. But the root problems, or those people that start these small chains of reactions are why people get mad, and get "toxic".

Have you ever played a game of football (not the american handball version) and had a person on your team just start purposely passing the ball to the opposite team everytime he got it? there are 2 things that would happen either A) they would never get a pass from any other player ever again, or B) they would just get kicked off the team entirely and told to get lost. Then usually because people arent inherently assholes, the other team will most likely sit one of their players out to make the game fair in terms of numbers again, that player would then tag in/tag out with another teammate and things generally go smoothly from there.

The problem is that it is difficult or perhaps impossible to implement the same kind of thing in League, get a teammate that feeds/trolls/afks? you cant just tell them they arent welcome in the game anymore and kick them out, likewise you cant leave the game yourself (not that you'd leave a football match because of one person being an ass either!). And of course, the enemy team will not and cannot just "sit a player out", so the player(s) get frusturated at the audacity that a player has to ruin the game for them BECAUSE THERE IS NOTHING THEY CAN DO ABOUT IT. This kind of behaviour would not fly in the real world, people just wouldnt think to pull the kind of crappy behaviour they do online because the computer screen between them and the rest of the HUMANS who are playing the game for some reason makes the logical part of their brain check-out for the day.

Yes, you can report them after the game the same way you would tell all your friends that X person was being an unsportsmanlike player that would ruin a perfectly fun and fair game for them by passing the ball to the opposite team on purpose. That person would then quickly be blacklisted, this is a permanent "punishment" or consequence of their actions because they only have ONE identity, unfortunately, there is no way to institute a similar consequence online as you can make infinite identities/accounts/aliases.

Is the root of this problem treatable? hard to say, unless some more extreme and unforgiving measures are taken to dissuade persons from doing such things it is unlikely it will ever stop.