Why does Riot even ban people?
Because your behavior is not how most people act, most people don't have tons of throwaway accounts and will learn how to behave after as little as 1 punishment.
You have to know by now that people just create a whole new account and all you are doing is making it worse for new players.
That's the problem with F2P game models. Anyone can make a new account for any reason and the only way to curb a specific person from doing it is if they're notable (such as a streamer or content creator) or if you have to verify accounts with things like SMS or ID numbers.
Although I should mention, Riot isn't the one making things worse for new players, the permanently banned players are the ones doing that. It's not Riot's fault that someone chooses to come back and berate new players, they're choosing to disregard the clearly laid out rules and because they disagree with them, they think they have a right to be angry and take it out on others.
It's an endless cycle and the only way for someone to break it is to be self-aware and act maturely.