the entire point of the game is to win, you failed to win. You get nothing. You make MMR more based on performance and quickly it'll devolved into "i am doing well, my team is not so i am not going to bother helping my team win since i don't actually give a damn about winning" and there would be more and more people being insanely annoying when they get 'KSd' or when someone gets their CS even though they do not need it. There is also the fact that if someone is so far ahead then all that will happen is that whoever has this huge lead will just stall the game to pad their stats instead of pushing their advantage for a win.
Another thing is that you can be 10-0-0 and someone can be 0-10-0 and they can still have the same impact on the game. Kills don't mean anything in the grand scheme of things, kills are nothing more than one of the many means we have to kill the enemy nexus. Good stats don't say much without context. You can be 10-2-0 on a game you lost but we do not know if it was because you were splitpushing the entire time, killing the same dude over and over, never grouping with your team, never assisting your team, never doing anything but splitpush because that is all you care about.