If the game has been slowly moving away from solo carrying, why can 1 player lose you the game?

Riot has implemented a lot of ways to make the game much harder to solo carry, in other words, 1 person outright winning the game when they get ahead. I actually think this is a really really good change. However, there is still a problem where 1 player can lose you the game, and yes this can happen. The idea that you win as a team is generally true, but losing as a team, not so much. I think it's a good thing 1 player cannot carry 4 people, but shouldn't 4 people be able to carry 1 person? I just played a game where we had 3 winning lanes; mid lane, jungle, and bottom were all winning pretty handily, but not by crazy amounts. Enough to be a pretty strong lead. Our top lane however was down 80 cs and had 6 deaths by the 10 minute marker. Our leads all across the map were virtually irrelevant because he was dead weight. We lost the game because we essentially had to play a 4v5, even though eveyrone else had nice leads, we couldn't carry someone that far behind. While I do think playing poorly should make it difficult to carry, I find it patently ridiculous that a single, extraordinarily bad player can lose the game for you by themselves. If I can't solo carry a game, I also don't want to be able to single-handily lose the game for my team nor do I want anyone else to.