If you reduced the LP loss after someone went AFK or fed terribly, you would be promoting some very bad behaviors.
First for AFK:
Once the game is considered "lost" everyone who wanted to "protect" their LP would switch from focusing on trying to find a way to win to focusing on making the game as unpleasant as possible for someone on the team, to encourage them to AFK. This would cause a cascade effect, as once someone else notices the first player's behavior, they would also be encouraged to find someone to annoy.
You have already seen some players call the game lost after first turret....first kill...first summoner usage....first gank....first 5 cs deficit...the end of champ select...the ban phase. The games would be very likely to be decided by the people who are least capable of actually knowing what the game state actually is.
The prevalence of throwing the game to protect individual LP would also prime everyone to "detect" that behavior, causing the cascade effect without a catalyst.
Now for feeding.
Once the game is considered "lost" everyone who wanted to "protect" their LP would switch from focusing on trying to find a way to win to focusing on increasing the death count as much as possible for someone on the team.
No more wards, no more assisting in teamfights, no more roaming to extend leads (I need to protect my KDA, and if I help them, then they won't get flagged for the loss), no more supports willing to die for their ADC, no more picking Leona or Amumu, no more feast or famine champions at all.
Those games would not be fun.
There are many many facets to contribute to any given team's chances of victory. If you're matched with people who have a very obvious flaw, that means that their other factors must be above yours, otherwise they would lose more games than you and be ranked lower. When you see someone with flaws, look to them to learn where you can improve. If you can't see anything to learn from them, that either means that they're having an off game and their performance this game is not indicative of their skill level OR it means that your flaws are on facets of the game that you don't even know about.
[{quoted}](name=Silent Gravity,realm=NA,application-id=ZGEFLEUQ,discussion-id=GJGZwq0W,comment-id=0002,timestamp=2018-09-22T23:06:38.485+0000)
Matches are made based on MMR. Everyone on both teams is in a very small range of each other's MMR. MMR varies from around 60 to around 3000. (Your MMR is approximately 516-616, for reference.)
Imagine that your teammates and opponents were all 60 MMR. At that level, it is unusual to buy items, cs, or go to the proper lane. You would fairly easily be able to solo carry every game. There would be very occasional losses, but your high winrate would increase your MMR. You would soon face more difficult opponents. As you faced stronger and stronger opposition, your winrate would slowly decrease. When you contribute the same on average to any given team's chances of winning as everyone else at that MMR, you would stabilize with an approximately 50% winrate.
Imagine that your teammates and opponents were all 3000 MMR. At that level, build order, back timing, wave management, csing, trading, vision control and all the other things that go into being good at the game are being done very well. Your opponents would very easily exploit your mistakes, and your team would generally lose. There would be occasional wins, but your low winrate would decrease your MMR. You would soon face easier opponents. As you face easier and easier opposition, your winrate would slowly increase. When you contribute the same on average to any given team's chances of winning as everyone else at that MMR, you would stabilize with an approximately 50% winrate.
