why do you get the worst teammates on the best champions

Almost every single time I get syndra I get a 0-10 botlane and a boosted jungler this time its a support thats duo with jungler no stone 0-10 lane. A jungler that some how wins games on amumu despite not being able to actually engage at all or ult during the fight. Goes to show how wonky the matchmaking is this guy can have a positive win rate while having a 30% win rate with his duo and playing his champions at a silver 3 level LOL