How SBMM actually works
Skill based Matchmaking is not how you think it is. It's not about how good you are it's about how much zu play. Let me explain.
Let's say that you are a skilled player and after a few good games "SBMM" decides you should lose your next match. It will pair you with below average teammates also due for a loss and stronger opponents/stacks to reduce your odds of winning. It is NOT about making the game fair.
The system rigs your to make you win or loose in a peticulare pattern to keep you playing as much/long as possible. For example: if you have bad teammates and have a bad round you ceep getting bad teammates because you ceep playing. If you have a bad round but stop playing for a while then in the next round you will have better teammates so you have a better round and ceep playing.
So a decent player with 1.x K/D who ceeps playing even tho he has bad round ceeps getting bad rounds, while a trash player with a 3.x K/D who stops playing after he had a bad round will get a better match next time.
I hope now you have a better understanding why SBMM isn't working.