Humans are VERY inconsistent
Having someone who does poorly on your team isn't Riot intentionally putting a feeder on your team. For all you know, that person could've been on a 4 game win streak completely carrying those games all by himself, but then this game he just happened to get put in a bad matchup or made a few big mistakes early that screwed him over. Of course there are times where a player is just really bad or boosted, but everyone occasionally gets those players on their team and the matchmaking system can't just not let them play the game.
Just so you know, you are also the garbage, feeder awful player that shouldn't be allowed to play ranked at all in some of your games. Even if you're a good player you'll still have those games sometimes. You could also just be a bad player that refuses to improve, but according to most people they are absolute Gods at this game that can do no wrong and the matchmaking system puts intentional feeders on their team in 100% of their games.