Pretty much what Sai said. I mean its pretty easy to see with the new ladder, people at 2400+ without at least a thousand game played this season are few and are mostly alts of people already at the top on their mains (valid for people with 300-400 games spread on 3-4 alts like Verdantstorm for example). Practice makes perfect.
There's other factors though.
I've been playing around the duelist bracket for many seasons and i consistently face the same problems, a lot of people around 2k-2300 tends to develop massive egos wich means they just won't queue with anyone they don't consider "worthy" wich greatly limits their practice sessions, they also tend to sit their rating forever to either get rewards as they're scared to tank or simply out of pride to not drop below what they consider "good" rating (usually milestones such as 2k and 2200), on top of that, most of those players actually stop improving there because they start to think they're too good to lose, so comes into play the endless amount of excuses on how the game is shit, how whatever comp beat them is skilless, how XYZ class is OP etc and they start raging and disbanding teams/dropping partners if they ever lose to teams they don't consider "worthy" based on above criterias, then they start the process all over again with other people. All of this completly shuts down any possibilities for self analysis required to improve your play.