Why is CS so important to the grade system?

I am an Irelia main and I have been trying my hardest to get an S/S+ for the level 7 Mastery. I am starting to realize the more I farm the better my grade. I am a team player and I don't like how much I have to farm because that means it is harder for me to carry games. My first S+ was a game where I went 16/4/4 and the reason I got it is because all I had been doing was farming, but even though I was farmed up and ridiculously fed we lost the game because I was stuck top the whole time. Recently I played a game where I went 19/2/11 it was a 37 minute game where I had 200 farm but I won, not because I was fed but because I was able to help snowball other lanes. I also had the most kill participation/gold/damage/least deaths. I got an A for that game. The point I'm trying to make is yes, farm is important but I don't feel like it is more important than helping your teammates, and this system encourages afk farming top which contradicts Riot's policy on a "team based game". This is just my opinion on the matter, and I seriously hope it changes because Irelia isn't the only champion I'm gonna want too master and it is hella easy to farm with her.