I think there's 3 big problem: dungeon philosophy, SE and people themselves.
1) The dungeon philosophy because even if they are making them more "varied", nothing punishes you, so you don't learn. Look at how many people at lvl 80 still not having any idea what to do with a stack. Most bosses actually work the same way, and I think that's why people find them bland? i'll do the rundown a bit lower.
2) SE because they could provide an in game encyclopedia with the most common markers (stack, eartshakers, red marker, tethers), a basic rundown of the mechanics of each dungeon after clearing them (like wow did) or even basic role/job guide, as they are doing it as a paper guide they sell anyway. On top of that, because you can be banned for just guiding someone if they get pissy, that doesnt really motivate anyone.
3) And the players, because they don't want to put effort in a game? (you don't pay my sub syndrom)
It's sad because every dungeon is basically: Aoe every 30/60 seconds, followed by a tankbuster nearly 10 seconds later or inverted. And people legit find that hard to heal. After that it's basically a mechanic, very casually shown, then the aoe, the tb then back to the mechanic but "with a bonus" and that's it. And they do the same in raids, look at the latest nier raid, second boss, kefka syndrom: they show the left/rigth mechanic, then they show something that makes it a twist, but if you use basic logic you can guess very easily that it's probably the opposite?
I just can't understand how people can find the actual dungeon design to be hard after you understand that?