Do you want to know why some roles/champions are deemed as unsatisfactory? Why toxicity exists?

Because of your behaviour as players. And not only in the extreme situations, when you got to play with feeder, afk or trolls. It is the attitude of a tons fo players, that make people lose time and games through their terrible behaviour and lead to the rise of toxicity in the vast majority of players that would have normally been positive or just neutral players that want to enjoy the game

Even though losing because of trolls, feeder, afks can lead to rage, the most frustration I get when playing league is when there is no feeder, troll or afk in my team but just a bunch of spoiled pricks (even worse if packed in a premade - but solo players aren't necessarily better -) that literaly refuse to listen to your advices for 35 minutes and thus proceed to lose you a game that would have been easily winnable if they were only weren't just idiots. You can find a somewhat acceptable cause with the "extreme" situations and somewhat control your rage, but when you lose because of no real cause other than people being idiot is a terrible feeling. And this is super frustrating especially when you got on a role with less resources and damage to carry like a tanky toplaner/jungler or a support, these pricks are the resons why such awesome roles are despised.

You can't win alone in League. To win in a league game the bare minimum is 2 players playing well, one that carry super hard and at least one that can help him in a competent way, being either in setting up vision, shielding for him to have him safe while unleash damage, being able add the extra bit of damage to make the carry complete the kills, securing objectives. It is enough that the others just don't do stupid stuff to let the enemy win (often you would need also at least a third player to actively help, but let's just keep things simple for the sake of discussion) The problem is that if both teams have those two potentially good players to win, and you are one of those 2 people in your team as support/tanky top laner, and your stupid teammates believe that they don't have to listen you because of that stupid misconception that you are a lesser player, you are done because you don't have the brute power to overcome that and this lead to huge frustration and exchange of toxicity.

Look at it from support perspective for a second: You are really far worse than me at shotcalling if you are playing in a freaking carry role and you end up in my team with the same MMR, you MUST listen my orders. It is not a request, it is an order. Or at least, explain why you are not following the order of your superior in the shotcalling hierarchy and why you want to do something else, because I could be the one making mistake

This lead people off from playing support role that is otherwise awesome, because you can feel the power of channeling the game in your team directions by not sheer mechanics but just by the use of your brain Of course, supports and tanky guys are not saints, and the above situation can occasionally be the exact opposite with the carries being left alone by the stupid teammates. But generally the roles that suffer are the ones with the lest direct power, and everyone, in every role, has to recognize the root of the problem, because acting as intelligent players in game can only help everyone enjoying the game more AND increase the satisfaction in playing certain champs/roles which in the end can lead more people to try and enjoy those role thus also solving problems with queue/matchmaking which gives more balanced games and so on feeding a positive spiral