Ranked

Why is it that riot gives and takes lp solely off wins and never off how you preform? Lately I’ve been playing support and I keep running into troll adc’s or people who never play the role and decide to try it for the first time in ranked. I’ve helped carry them in my lane, poke them down so all they have to do is auto attack and we smash lane. But because other lanes feed we end up losing the game. I have some games where as a support I go 10/0/14 but because we lose, I lose 20 lp??? How am I supposed to rank up if other people are trolling and I’m unfortunately queued with them? How come no matter how well I personally preform it does not matter whatsoever to my rank. If I go 0/20/0 and we win the game do I get 20 lp for that game? Does that make sense at all?