LP Based on performance

After my last win I received 18 lp and then moved up to 64 lp. I do the math Okay I need 2x win for 100lp. The next game I go 18/2/18 and win the game with a s+ Grade and received 17lp. That discouraged me from playing my next game because I know that I will now only be moved up to 99lp. The question I have is WHY DONT PLAYERS RECEIVE MORE/LESS LP BASED ON GRADE PERFORMANCE? I do not know how many games I've been in where I am trying my but off performing well but still manage to lose the game due to one of my teammates not trying at all. Then im penalized the same way as they are even though I put up way more of a fight. Or vice versa when you still manage to win the game then are awarded the same exact way as your poorly performing teammate who did not really deserve to win. I believe that the LP should not be significantly more, but atleast something a little extra 1-5 extra lp if you managed to perform well. I believe this would motivate players to perform better each and every game, also making it more rewarding when you do perform well. On this topic as well there are games when 1 player leaves the game after going 0/3 in lane and the teamates lose lp. If 1 player leaves teamates should if anything not be punished in an identical way as if they actually lost the game. This is only my second season playing league, so I'm sure there are more people with a better understanding over this topic.