I semi-troll on my smurf because RIOT gave me a BS 2 week ban as a first offense on my 6+ year old main account for a single bad word. I won't troll on that account because I don't want to lose my stuff, but if RIOT ever permabanned that account and took my stuff the way it does to a lot of people in these forums, I'd start making accounts just to troll to hurt their business because I thought they wronged me. Most people don't troll for this reason, but there are some. Psychologically speaking....the worst of the trolls are psychopaths and sadists and truly enjoy hurting other people and ruining their fun. This is why people say don't rage at the troll, because that is quite literally what the troll wants you to do. That is what they enjoy, having the power over you. I'd say between psychopaths, sadists, and people who are bitter at RIOT, you have pretty much 98-99% of the true TROLL/GRIEFERS covered.
Now to answer WHY they do nothing. Well if you listen to a lot of people in these forums it's because "it's hard to detect", but let's be real that is bullshit. They could have humans review games and easily see who is truly int'ing versus who is just having a bad game.
They don't get rid of the trolls because they will just make more accounts and troll lower level new players....wait...but by that logic they wouldn't ban the flamers either because why would they want their new players having to play with flamers...oh wait, they do permaban flamers just not trolls. Proof that Riot cares more about flamers than trolls. Now why this is...who knows? Logically it doesn't make any sense, both are horrible to play with, especially for new players, flamers probably worse for new players actually because they won't know when someone is trolling for a while....so if we are being truly logical it would be better to ban the trolls and not the flamers because that will disrupt the new player base less....
IPSO FACTO - Riot makes no sense. Their support is the worst support I've ever encountered. They make people change names that aren't offensive... They ban people for saying words without taking context into account, but they won't ban the trolls who actually ruin the competitive spirit of the game. God forbid you say something to this guy who has now ruined 3 of your games and roughly 1-2 hours of your life...that you will never get back....he's allowed to do that but you aren't allowed to tell him to go fuck himself. Riot has it backwards friend... and a lot of their rulings just make no sense. It's kind of like being a child again and being told by your parent ..."because I said so".... even though your parent makes no sense they make the rules.