The only real answer is the game’s reward structures don’t resonate with them, or more so, they don’t enjoy what the game’s purpose is at max level.
You have multiple infinite progression pathways, and thanks to how lockouts and various types of content works now, you can just do whatever you like best over and over again all day every day.
That’s what I do. I do about 25+ Mythic plus dungeons a week across usually 20-25 hours. I would do them if they gave no loot, like I did with Challenge Mode dungeons in MoP and WoD, but the fact that they do give extremely good loot makes me do them even more, on more toons other than just my DK.
In the end, it is factually wrong to say there is nothing to do in the game. People who get bored by so many different types of content in a game this big should probably just start asking themselves what it is they actually want from this game, because if a comment like you see above where you just go through 5-10 different things to do and say you dislike all of them is happening to any of you, I don’t know what you are expecting.
This stuff is what the game has always had, and now it has even more.
I think the two big issues are that there is honestly TOO MUCH to do, such that you can never really get everything you want done in a week, and it feels kind of pointless trying after a while.
And obviously some people still seem to be playing a class they dislike, so that is contributing to problems there too for sure.
I understand if you played a spec for a decade and now you dislike it, but I was the same way in Legion.
The horrible changes they made to Holy Paladin had me ready to quit the game.
But I switched to Blood DK and honestly it was the best choice I’ve ever made.
Try new things. You’ll find something you like for sure. Best way to stave off boredom, regardless of what is causing it.