That doesn't necessarily mean the ceiling is high as much as expectations and accessibility of most content is very low.
It's a side effect of the game teaching you nothing and giving very little indication of performance. Most players have no idea they're 9th percentile. They're fairly sure they're doing "fine". They've sailed through the MSQ, beaten everything on normal and even been carried through a few EX trials. In their eyes they're doing well. The concept of uptime, correct rotation, melds, food, actions-per-minute, weaving, clipping are all foreign to them. Since everything is easy they don't have to try that hard. Nothing tells them they really aren't prepared for Savage, naturally they'd expect it's the next step, so in they go.
For a good player, I imagine they hit the ceiling too easily. Once you have the fight mapped out in your head, you get to a point very fast where the content is on farm and you're just trying to squeeze 1 or 2% more on your logs. A healer for example, has their heals planned for the raidwides then the rest is 1, 2, 1, 1, 1, 1, 1, 1
Tanking too is a good example, I remember years ago when WoW was still decent, I had such a wide tank toolkit that every time I died, even in a complete messy disaster of a situation where someone accidentally pulled 2 extra trash packs or all the healers died, even when I'd done far more than the average tank already, I could look back and say "I still might have lived if I'd rotated this, this and this cd, used that self-heal there instead of there, held this for this point, remembered and used that more". And I enjoyed that, there was always room for improvement. It was so rewarding playing well, you could do amazing things.
In ff14 it's. I died, but I did use a tank cd. The healer fell asleep/someone failed a mechanic. There is nothing I could have done. Oh well.