The matchmaking is likely rigged to force you to waste time, actually.

I can gather only one thing from this: Riot throws 'filler' matches into the system. Initially, that sounds kind of weird; It would make no sense for Riot to artificially inflate the numbers of matches played... right? ...well... it actually does make sense, from a purely corporation-oriented standpoint. More matches played = more time spent, and more money spent as well because LoL has to remain F2P or it loses a majority of the consumerbase. The more time people spend playing, the more of their life is invested into the game and therfor the more likely they are to fall prey to the sunken cost fallacy, and spend a bit more.

Notice how after 20 matches, almost on a regular basis, I go on a random losing spree despite doing nothing any differently. I'm not tilted, I'm not even mildly irritated, I'm just curious as to why my Mid is suddenly a Bronze 2 with a 27% winrate on Rammus Mid w/ Ghost, Teleport and Bounty Hunt- oh. This is not a coincidence. The only way to bypass this dumb and arbitrary limiter is to either carry a game alone (which Riot is mysteriously less keen on people doing now) or to use sites outside of that framework like OP.GG or LeagueOfGraphs (which Riot is keen on shutting down) and dodge when you see an inevitable loss.

The alter theory is that Riot does try and force a 50% winrate on you because a constant cycle of boom & bust appeals to the baser nature of humankind and acts as a comfort to them despite it causing many problems people bitch about constantly, which... if true, the previous two ways of bypassing it still apply.

One way or the other, no matter which theory is true, one of them likely is. This, mixed with the needless promos, means that you can actually come to see why some people will be stuck in Bronze despite being quite a bit better than their opposition. You can head into your promos once every 5-10 wins, which... obviously, means that eventually someone will head into their promos during their inclined losing spree. You fail your promos, lose 15-10 more and maybe get your MMR dumpstered, and now have to spend another 20 games bringing your MMR and LP back up to snuff... if you weren't demoted.

Moral of the experiment? Dodge games frequently. I know it's annoying, but if Riot is inclined on having a system this dysfunctional for sake of making people play more, then I'm not going to shed a tear that someone thought dodging the game with the "it works guys" 0/32/4 Rammus Mid.