Why is this a thing

I would give a picture but whats the point. I don't know why no one talks about this anyway but it's been something I've thought for forever. Just recently I went into a game and the enemy mid was so fed out of laning phase it seemed it would be impossible to win. After a few teamfights with Taric ult the game was turning around and it seemed like we would win since the enemy team was playing horribly.

And then my team threw and got picked off. I got an S in that game.

Then I lost 19 lp. I don't understand how that is fair in anyway. Fine make me lose lp but you're punishing me in rank for doing well. That's a system that makes no sense. At least make it so that if you get at least an S- some of your lp loss is diminished because to lose lp for doing amazing in a game makes no sense