Hey there! I'm the designer in charge of Ranked, so I can speak pretty directly to this. To start, I totally get the pain you're feeling here - seeing players with really different ranks in your game feels like garbage, even if we tell you that the MMR makes it fair. We want to solve this problem by getting MMR and Rank closer together, something we'll be talking about more over this season. But there are some really good reasons we don't just show MMR:
First, check out this post from a couple years ago where Riot Gortok goes over this question: https://nexus.leagueoflegends.com/en-us/2018/02/dev-matchmaking-real-talk/ . I think that answer holds up pretty well, but I'll also give some of my own reasons. Strap in, this is gonna get long:
MMR is meant to measure your progress, not display it. For MMR to measure skill correctly, it has to do a lot of things that, if you were watching the entire time, feels pretty awful. The easiest example is a new player. When a new player enters the system, we don't have enough info to match them correctly yet, so we put them close to the middle of the pack and move them based on results. But if that new player is worse than average, their MMR starts at one point and then drops. And drops. And drops, until they reach a fair match.
But if you look at how that player is playing, their MMR is not reflecting their story. They're typically getting better from game to game and improving - which is part of why we have Rank start at the bottom and move up, to eventually catch your MMR. And the answer here isn't just to have MMR start at the bottom, because that would just mean that low MMR was a constantly whirlpool of actual low MMR players and higher MMR players moving their way up, generating a ton of unbalanced matches.
MMR needs to change quickly, Rank does not. For MMR to work correctly, it needs to be both fluid and responsive to loss and win streaks. This means that if you drop 4 games in a row, even if it was just because you had some bad luck, too bad - you're dropping down. Sure, it'll go back up if you keep playing games, but hitting those troughs hurts a lot. Similarly, you might go on a lucky streak and fly upwards - only to get stomped a couple times and fall back down. It can become very hard to say what your MMR "is" - MMR stabilizes over time, but it jumps around a lot more than Rank does.
With Rank, we can include things like demotion protection and promo series to make your rank more stable. Loss streaks aren't as punishing, and win streaks do less to inflate players' ranks.
Showing MMR doesn't address all the concerns with matchmaking. MMR is only one component we have to consider with matchmaking, including who is looking for a game, how long everyone's been waiting, who is grouped with someone else, etc. Showing would give more information, but still not the complete story. Look, I clearly love giving overly long explanations as much as the next guy, but even if we gave full context on why every match was made, people would still disagree with our reasoning, because everyone values things differently. Someone might be OK with waiting in queue for 15 minutes to get their perfect match, and someone else might think that any wait over 30 seconds means it's time to quit and play something else.
Hope this helps explain a bit more, and I just want to emphasize again - we agree that the difference between MMR and Rank is something we can improve. We're going to be making a lot of tune-ups to Ranked over this season and the next, and that is certainly one of the issues at the front of our minds.
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