personal rank or team rank?

Why is it that our personal rank score is based on the win lose ratio of a random assigned team. i could understand this if we all played as a ranked team 5v5 every game. But we do not. We play a 5v5 ranked with players we do not know. We have a score system in place for after the match, it shows from s to d. Why cant it be that a player that gets a score of s in every game from 5v5 solo que with all loses get ranked better than bronze? if you are getting a and s scores on a regular basis, clearly you do not belong in bronze, but your win lose ratio shows that you are getting teamed up with players that do belong in bronze. I personally feel that solo que should be ranked on your scores, not your win lose