Just because you go 11/0 in lane doesn't mean you can't be responsible at all for the loss. Yeah, you might end the game 11/3/2, but the fact that you went 1 for 1 every teamfight while eating all the sidelane farm, jungle camps, and kills means you are a huge reason your team lost the game, not just your 0/6/5 mid laner.
Like for example: just a few games ago I was playing Morgana support, and our whole team was feeding and bot lane was pretty much the only reason our team got back into the game.
Yes, we got 4 manned 3 times before 10 minutes. Yes, the game might've been easier if our mid or jungle had pressure. However, we started to make a huge comback off some really good picks and teamfighting.
While the momentum was in our favor, I saw Akali top lane, and we were fighting around drake (elder?). I flash in to get a 4 man morg ulti, and their whole team literally just flashes forward and kills everyone on my team and I'm not in position to peel or use locket. Akali got back from base faster than I thought and cleaned up our team.
Could I blame the mid/jungle/top for feeding? Sure. But in the end even though I was probably the best performing member for like 35 minutes, I single-handedly lost the game.