I've been gaming since I was 9, back in 1983, and to this day I don't "get" mega-dungeons in D&D. I've had people try to explain it to me, and I've just. Never. Gotten it. I find even normal-length dungeons boring more often than not.
So, I'm going to try to ask it a different way.
What do you enjoy in a mega-dungeon? What do you get out of it that you can't get--or at least not get as much--in other D&D games/settings? Why do you enjoy those aspects?
(Important background: My tastes run to plot-heavy games, where the combats are almost all plot-related; and to fewer, more dangerous combats. I don't care for random encounters. I don't care for combat for its own sake. I prefer mystery-solving and social/political interaction to seeing what's in the next treasure chest.)
I don't have an answer for you. I like combat.
I'm guessing a dungeon wears out its welcome after a while.
Either it keeps to its themes and presents more of the same (which loses my interest) or it offers incredulous variety (which gets so unrealistic it too loses my interest).
I simply see no need to stay in one and the same dungeon when it's so much easier to place a different scenario somewhere else.
In the end, I view the dungeon as playing the game with the training wheels still on. Meaning a dungeon has its place: its limited nature makes it easy to DM. (A regular sized dungeon, that is.)
But after a while, you move on. Then, dungeons become more of a trope you use only when really appropriate, and not otherwise.
In 5th edition, a dungeon is a good way to make sure the party is softened up by a few encounters before the main BBEG-facing event.
Even then, I see few reasons to use
large dungeons and none for
mega dungeons.
I do see the attraction in using a dungeon for combat-as-sport; that is, making it a game challenge to survive the dungeon. But even then, there's a limit to how big they need to be. I mean, I imagine even Dungeon & Dragons Online (surely combat-as-sport if there ever was) videogame sticks to medium-sized dungeons; far from the "mega-dungeons" of this thread.
So, no.
I've had a DM who villingly selected and ran things like the 3e Temple of Elemental Evil for us, and boy, did it get dreary after a while, especially when we realized we essentially acted as nothing more than cleaning ladies (except the office was a volcano ring, and the dust and spilled coffee was monsters abd cultist priests)... le sigh.