Why is matchmaking broken

It can't be just me experiencing this...

I can win 12 games in a row, skip an entire rank, all the way to S1, get into promos then all of a sudden, doesn't matter how I play or what I do I lose back down to S5. SEVEN (and not exaggerating) SEVEN times this has happened. There's NO fucking way that the people I get matched with are the same mmr.

"You're a Silver player blah blah blah" then why is it I can walk all over "silver" players 12 games in a row (consistently until S1) then all of a sudden everyone on my team loses lane so bad to the point I could afk in base and it wouldn't change anything. No matter how good I do. There's no way that 4 games in a row someone on my team goes 0/8 and have the same mmr as me. Something there ins't right, and it's not fair I lose LP because of this.

It's not my bad playing, otherwise I wouldn't have those win streaks. Nothing about my playing changes but all of a sudden when I'm about to get promoted my entire team get's stomped on. EVERY, game.

I've seen numerous posts about this and it's really annoying that this happens. I VERY rarely lose my lane. It's not my playing.

Either smurfs, afk's, trolls etc... But it only happens when I'm about to get promoted.