How do you rate exceptional performance, then?
And how do you do so in a way that doesn't distract from the main objective, and treats everyone, including their strengths and weaknesses, fairly?
You could go with KDA, but that just encourages kill farming instead of dying for things that are worth the trade or taking objectives, and doesn't always match up to actual performance (As the guy who got camped but still mitigated losses will lose more LP than the guy who played super passive against a Nasus). Objective damage would be an option, but that's massively dependent on how the game goes, the champions picked, and the runes used, and means the guy who did poorly only because he got camped loses more than the guy who somehow lost lane after his opponent showed up two minutes late.
There's just no actual metric you can compare it to that's fair for everyone, and doesn't mean someone loses more for being camped or for playing for victory than the guy who actually lost the team the match did.