I just encountered the same toxic player for the third time in a month: what the fuck, Riot?

And unsurprisingly, they were still toxic. For added 'wtf' value, two players on the enemy team knew this player was toxic from past games.

This person has over 1300 ranked games played this season alone. They have not been banned. I've reported them each time I had them in my game, and the two players on the other team said they also have reported them before.

How the fuck do you fail to ban a clearly toxic player (i.e. raging in chat, intentional feeding, 'gg open mid im afk') who has been at this for over a month, and likely much longer, considering the number of games played?