League is for fun
League is a video game. It's sole purpose is to bring enjoyment and relaxation to those who play it. Not wins, not massive KDAs. Enjoyment.
The vast majority of players in this and other games don't enjoy the game if they see shaming, aggression, bossing, or other types of flaming. It doesn't matter if you're losing the game. Enjoyment does not hinge upon winning the game for most players.
So if you're flaming, you're ruining the game for others. And many times you're ruining it worse than an intentional feeder. Because look at it this way - when you go into a game, you expect there's a 50% chance you'll lose. You're ready for a loss. What you don't expect is someone to be flaming or harassing others during the game.