Imagine every other player in the game is a random 50% coin flip. Obviously they're not but just bear with me.
If you play at 100% consistency, your team will be at 300% whereas the other team will be at 250%. So even if you're destroying every single game, you won't have an insane winrate. And I'm guessing that you don't get fed every game, most people don't.
It just takes time. As long as you're playing with better than 50% consistency, you will rise in rank. It may take some time, but half the time you'll get teams who are good enough to win. And if you play better than everyone around you, you'll win more than half the time.
tl;dr it takes a lot of games, don't focus on the ones you couldn't win