Why isn't 15+ deaths immediately removed from ranked play?

I feel like no matter your ELO you are facing people of your own skill level. But when you are just walking into lane and dying again and again to the point where you die 15+ times, i feel like you just know nothing about the game in general and shouldn't be allowed to play ranked. Not only that, they can just say they had a "bad game" and get no punishment for doing so damn poorly. The rest of the team, does seriously well despite the 6k gold handed to the enemy team for free. So my question is this, where do we draw the line on how bad you can do before you just get instantly removed from ranked play. It isn't fair to the rest of the team to lose LP and 40 minutes of their time just because of one person who can't use their brain. It just ruins players game experience, and its the one thing that just isnt punished for some reason??

Also how is this person so outmatched skill wise that they are dying 15+ times in a game?? Feel like the ranked system might be broken for matchmaking atm and needs to have a fix... Perhaps stop matching based on W/L and just go with MMR like every other ranked system in existence.