Where is my Lp

Why do I lose LP when I have a disconnected player the whole game? It is extremely difficult to win 4v5 games(not impossible but very difficult) and it puts you at a huge disadvantage (less pressure, damage, cc etc) so why do I lose Lp because someone else disconnects and pretty much loses the game for us. I feel like we shouldn't lose Lp when someone disconnects from the beginning and we should gain a huge amount of Lp if we pull off a win. I also think that after you go to promos 5 times you should automatically win your promos (not so sure about jumpng a tier though), you obviously have the skill to progress higher in your tier, but fail to win the 3 games in promos.