Leaver/Ragequitter lp loss

Why is the burden of full LP loss from one asshole whomst brain implodes placed on the entire team of 4 other people. Why are leavers/ragequitters not penalized the majority of the lp lost and it lessened for the other teammates that are forced to sit and slowly lose. The odds of winning a 4v5 are drastically low and you all know this. So I ask, why is 1 person that obviously isn't concentrating allowed to decide the outcome of a game for 4 other people?

If there is a system in place that cuts down LP loss(50% or more) for the players that are forced into a 4v5 situation on the leaver, how would it ruin or break anything. I am not trying to discriminate against people that plainly dc either, I don't think it would be that hard to implement a check at the nexus boom to see if a player is in the game or not, to avoid punishing people that merely just DC and try to reconnect. A system like this would rapidly derank these overly toxic players and make them the bearers of the wasted time and not the team they leave behind. I don't see how it can be abused at high elo to boost other players with alt accounts, because again, if the leaver deranks so rapidly they wouldn't be able to queue with the account they are trying to save lp for. Our games should not be decided by 1 person that thinks they are some gigasmurf, or as riot puts it, lord dgaf.

This system also doesn't diminish the reward for winning the game because the winning team obviously played well enough early to make someone want to ragequit, but the punishment for leaving a game as a 4v5 far before it is decided needs to be harsh in my opinion, especially in the mid tier ranks where it is probably happening the most.