https://en.wikipedia.org/wiki/Elo_rating_system

Same system League of Legends has too, roughly.

The general idea is to aim for a 50/50 match outcome. You know, fair and you're being matched with people generally around your skill level. Smurfs/boosting kinda ruin this to an extent. They can artificially inflate/deflate(?) your team's SR rating and you'll be matched against people you shouldn't be playing with.

Essentially if you do gain the knowledge, experience, and mechanics to get past a certain division, then you should be smashing your opponents into the ground. Thus winstreaks until you hit another wall.

However when it comes to team games with a matchmaker that just takes whatever pool of players are available(I say this in a general sense, the system tries to get as close to your average SR range as possible), then you end up with matches that can get skewed simply because there might not be people on at the time.

But I do have to note that the elo system was created for 1v1 scenarios. Which is why the rating system is kinda wonky for team games and why rating gain/loss can't really be super dependent on personal performance statistics. It's hard to gauge and interpret team contribution just based on medals themselves.

Hopefully this gives you some insight on why things are the way they are.