Why is matchmaking doing this?

So i just went to a ranked game (no losing streak, no winning streak) and got matched with these players http://prntscr.com/83vpfi As you can see in this screenshot i got a gold 4 rengar in my team even trough its plat ~ 3 game. He lost this game by himself went 2/11/2 and had no pressure didnt do drakes, no good ganks (he did some but they were bad and he just died) . I know that now i cant change anything but just asking why is this happening? How do i prevent this? And sorry if i made some grammar mistakes.
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