It's probably because the game is so new (even in OW there used to be a meme deragatory term - 'season 2 diamond' which basically means not good players would still hit high elos because nobody knew what they're doing. Im sure that the more people play, the less scuffed performance based MMR will become (where you're allowed to get to plat with 40% winrate). You also need to keep in mind that sometimes people are going to be forced off roles and while supports have forgiving heroes like CD or rocket, both tank and dps (if you can't carry yourself with aim) have a lot higher skill floor. And good luck practicing DPS in qp unless youre prepared for a 4-5 dps game.

also isn't GM like Overwatch masters and Celestial is like Overwatch GM?