High play/low diamond is annoying because there's actually insanely different kind of players in there. There are Junkrat mains who are naturally next to useless in attack, but are very good in defense resulting in them winning some of their games even tho they "main" a kinda silly hero, which is enough to keep them floating at that rank. Then there's also some players who occasionally play incredibly well, but it's not consistent. They have some very good days, and then some very bad days. There's a lot of otherwise decent players who just completely lack the ability to play as a team and communicate, which keeps them at that rating probably forever. There's also ton of people who basically just got placed there 3 seasons ago and they keep floating there playing 20 games a season (there's actually surprisingly much of these).
Tbh, it's much easier to climb out of gold than it is to climb out from plat when playing solo. At least gold players are mostly just bad, so when you personally play better, you WILL get out of there. In platinum you have to rely a lot more on your team. But don't get me wrong, it's still possible to climb out of there. Just probably takes more time, so the trolls and whatnot get evened out.