Law of large numbers. You take the spot of 1 player so if your not bad, don't afk, don't lag, make the smart plays, You will win more then you lose. Especially as the odds of your enemy team having one of those problems is higher (unless your the issue then your team is handicapped every game cuz your on it.) The Ranked matchmaking system rewards consistent play that's higher then your elo and punishes the opposite. Your skill as an individual player is the one constant that can be found in every game you play, regardless of teammates or enemy team. The odds are in your favor if your better then your elo. It really is that simple.
Edit, furthermore as its a team game a players skill is directly related to how well they are able to work with or around others. There is no value in making a new system that rewards players for playing well but no going for a win. It encourages selfish play and people trying to protect their stats rather then aiming to win matches