Why is it so hard to get S ranks on carries while actually carrying?

I got an A+ on this game. I know the CS for getting an S on Lucian is probably expected to be pretty high since he's one of the most played adc's, especially proportion-wise at higher elos, but how does my KDA not outweigh that? I get that CS should be an important factor in rating how a player did in a game, but why is it so important that it's impossible to get an S no matter how well you do without it?

I've had A+ games on Lucian where I was around 10/8/8 with 7.5 CS/min, so from what I can tell the current scoring system roughly equates that with a 27/3/15 game with 5.4 CS/min. It seems like the playstyle they are promoting for carries who want to get mastery 6/7 is to get to a reasonable KDA (ie. 10/3/7) and then just afk farm, and stop caring whether you win or lose. Rito...?

PS. Yes, the enemy team comp was ridiculous in that game lol