MMR is accurate but not 100% accurate as some people claim. Nothing is perfect and it has flaws too. I've seen many 4k players who should be in higher bracket, but they're not because cap. Even with 85%+ winrate, player who should be 5k will need to play around 70-80 games in bracket he doesn't belong in which is honestly boring. Sure, some will say if you belong higher, you MUST win all games in lower MMR but it's not possible to win EVERY match. Once in a while, there is high chance you'll meet either raging feeder, someone who feeds mid, someone who insists on playing carry and forces you to pick support, but fails to farm and makes you lose game or simply enemy team who has 2-3 players that are as good as you, or one that's better than you. You can't be only miscalibrated player in entire game, right?

MMR shows how good you are at game (basically your ability to win games). You might have 15-0-2 KDA but still lose because you didn't help your team.

MMR is not really accurate but 1k player who stays in 1k for 1 month of tryhard playing truly belongs here. You have to improve in order to climb. I know one player who calibrated on 900mmr. He rushed acc level without even knowing basics and ended up in sub1k, but he learned game better, trained and improved after that. Then he started stomping matches, so instead of playing in 1-2k to climb, he made new account and calibrated on around 3.5k, then climbed to 4k and now he plays where he belongs. If you want higher mmr, you must get better. Make new acc or just play on your current one, if you don't improve, you will stay in low bracket.

Tl;dr MMR is accurate but not totally. Everything has flaws, but you can't blame your teammates for not being 5k if you're 2k. It's just nearly impossible to have feeder team in every match.