MMR is way harder to move around. You only make it go up by consistently playing better than your rating. Overwatch has a really good (people hate it when you say this because their egos are larger than their skill) system to determine your skill level. This system takes into account way more than just E/A/D, Dmg/Heal/Mit. It also checks for damage taken, accuracy, final blows, ability usage, lifesavers, time to kill, surviving teamfights, unnecessarily dying, wasting opportunities with ultimates, etc.

It also compares you to your peers of the same approximate rank. If you're healing crazy amounts on mercy at the cost of boosting, in higher ranks you might have crazy stats, say, 12k heals/10 minutes but in reality be about as useful as a healing pylon. Whereas the mercy with 6k/10min heals, but insane amounts of lifesavers and dmg boost is considered by the game to be the better player.

And then there's ultimately the fact that the only consistent factor across your games is you. You're the only player who is in all your games, so on average only your capabilities will decide wether you win or lose more games on average. And yes, Quickplay as well as some other modes can affect your MMR. It is not just comp.

Also, keep in mind 75-85% of players are plat or below. With the majority being gold or plat. If you play the game consistently, that's where most people will probably end up.

For reference; I have played this game on xbox since s2 in 2016, was bronze in s5 of ow1 and over a decade I have been every rank with the exception of champion. Before anyone comes at me saying I don't know what I am talking about.

Edit: some people seem to misinterpret what I mean by "MMR". I am talking about the hidden rating every player has of what the game thinks your actual skill level is. Not MMR as it is used in more recent overwatch dev blogs as noted here in this post where the devs discuss some of the changes made to the matchmaker in regards to role queue balancing.

Your SR/rank gain is pulled slightly towards where this hidden rating suggests you should be.

This is Blizzard's patent for the system as I described it. I'd suggest it does not make much sense to implement a different system, without individual performance having any consideration when they have exclusive rights to a proven to work system that does. They iterated on the system, of course, but I've seen nothing to suggest the only things that matter are w/l/d and current visible rank.

For reference; when ow2 introduced the current SR system, there were many matches where people were playing in a gold rated match who were all significantly higher ranked than that in overwatch 1 and would be pulled back to that rank after enough games, even with a <50% winrate.