How does this even happen?

How do you lose a game where your team is 5k, 3 towers, and 10 kills up and group up yet somehow kill no one on the enemy team (during 3 separate teamfights) and lose the game. It was like the entire team just went "FUCK YOU, I deal no damage now". Honestly, I have no idea how this happens. I feel like if I knew I would be in Diamond division or higher. We pushed objectives after kills and were never wasting time. We won 2 teamfights in a row, overwhelmingly, and then...poof we just couldn't win anymore. Not only could we not win the teamfights we were lucky if we killed 1 enemy champion. How does a team that completely stomps the enemy team become so useless? Anyway, within 6 minutes we went from 5k ahead to 5k behind. Why? I will probably never know, they didn't even take baron for the extra gold they just... killed us all, all the time, even when we were surprising them.

sigh...