because you have silver/golds on your team too and they dont play like you expect them to.
i just had a similar situation in a normal. we just killed most of the enemy team, snagged baron and i didnt even look at the map, but automatically walked top lane after hitting tab to check enemy respawn timers, as it was the lane that was pushing in the most.
since we were all pretty much full health, i just automatically expected my team to walk down mid and take the turret and the inhib there.
suddently i got rolled over by the other two enemies who were still alive and one guy who respawned, was confused as to what was happening, and realized that half of my team recalled and the other half split up to do random camps in the jungle. no one even walked mid.
we lost the game when they got caught aswell, when we could have just ended it from our position. or at least we could have broken down most of their defenses before resetting.
low mmr doesnt play the same game.