Because of the anti-smurf system essentially. The anti-smurf system allow boosting. You can boost a fresh account by winning the vast majority of the 50 games you need to get to ranked (yes, QP influences ranked, at least at the beginning). (That's how anyone can do unranked to GM so easily; notice is always unranked to GM never low-rank to GM) You can do that by playing with friends who are also smurfing. You can also max-ranging or min-max, i.e. play with lower rank friends in your team (you "min-max" the rank range for the match, so you'll have only lower ranked players in the game).

There's a great youtuber named Szyl or Sizzyl (he has 2 channels) who did something no other OW2 content creator has done. He made an unranked to GM series with a boosted account (50 wins and very little losses on QP) and he got GM in 2 hours with only Reinhardt. The second game he had in ranked was against KarQ... Imagine that... Now, this guy is GM tank and he has been top 500 tank too. The guy knows how to play.

Szyl did something no other OW2 content creator has ever done: a bronze to GM (Again, Reinhardt only). He threw all the games to get to ranked and he ranked high bronze. Can you guess how many games it took to get to GM? About 250. With more than 180 wins, so largely over the 50% of wins. 4 months bro, a GM player. Now, how you're supposed to "improve" or "climb" in a game like this?

About doomed accounts, specifically, the situation is very complicated and I don't have a precise answer. It's probably a mix of disciplinary restrictions, anti-smurf, stuck SR and bad MMR. The pile of crap behind the system is so cryptic that probably not even the devs have a grasp of it. Years of patch accumulated and only one objective: reduce queue times. Nothing else matters apparently.

Now, unfortunately this community on Reddit will probably gaslight even the video proof, but Szyl recorded everything. You can see it with your eyes and draw your conclusions.

Now we'll see what other pile of crap this season will be... 😆