Why does individual player skill have such a small contribution?
There are 12 players in the match. If all twelve belong there, they only contribute 8.3% each. Only massively misranked people (or people that are playing very different than their typical skill level) can force the game to be won or lost.
The way for a normal player to overcome this is to play moderately better than their rank, and play enough games to overcome random factors.
See Bronze - Sliver - Gold ranking guide for more.