There are people who think that they have done nothing wrong and can’t accept it when it is shown clearly that they have done something wrong.
This happens for both gameplay and behavior stuff in league. The jungler flames his bot lane for feeding instead of looking for what mistakes he did and how he could have played better to stop bot from feeding.
With the jungler example: The jungler usually flames his bot because he wants to shift the blame to somebody else and feel better. Or He can’t accept the fact that he made a mistake or is wrong so he lashes out at somebody else (in this example it is his bot lane).
These types of people don’t really change that much and will always stay in the same negative loop, so it’s best to just ignore them (like muting and reporting him after the game) instead of arguing to try and make them accept their faults. You can help Riot show the toxic person the door (by reporting), but don’t get to a flame war or huge argument that gets both of you shoved out of the door (you get banned and so does the toxic person).
This was my reasoning, hope it makes sense and helped you understand why some people blame Riot or their teammates on a lost game.