Games are generally won when the most competent player is allowed to secure more kills versus valuable targets, while less competent players clear lanes and keep pressure away from objectives and their side of the map.
If less competent players take 80% of the best carry's kills, they average out the competent player's contribution, which makes the game more of a coinflip.
IE..... if you give my Jungle Zyra 10 kills, early, I will 1v5 carry you to the end of the game. If you feel the need to leave me 2 kills and 8 assists, we may still win, but my impact has been diminished by about half, and you may very well be a no contribution farming bot, or no objective player, which MAY mean I can't carry you.
If you leave me 0/0/18 Morgana, I may very well still be in the game, but since I fall off and you took my primary income source since Morgana struggles at last hitting, if you're playing a fall off champion as well, we very likely will lose, because I'll be behind a full item or two.
Unreactive teams. Ever have those games where you end the game with 20k damage but 4 people on your team were 4-6k damage, and farm was 2 cs/minute? Yeah. They basically stood back, let the other team take all objectives, whether they died or not, and didn't farm. It's happening a lot right now.