Ranked AFK'ers

So, I just played my first ranked game of the morning. I've been making progress up the ladder for the last week but I still have a long way to go. What bothers me however, is when people don't get the lane they want, or don't get the role they want, or won't talk in lobby chat to coordinate and cooperate with each other to manage a proper team. For example....I can play all roles, I may not be the best at all of them but to avoid shitty experiences I've been trying my best to keep everyone happy. This ranked game I just played.....I was third pick, the opposing team picked Tristana so I played Caitlyn. Nobody until that point had said a word about preferred roles. Our last pick was support (naturally because of everyone but me it seems hates playing support) and the guy picked Sona. That was great and all because we had a proper team until we got in game and the bitch decides to AFK, before the game even started. Naturally we got rekt and forfeited, but what I want to know, is why can't Riot implement a system where ranked leavers and people that AFK get banned to normals or dominion or arams until they've completed a certain amount of games without AFK or leaving?