It's been bewildering to me for a while, watching people try to reconstruct the epic dungeons of the ancient elders, or even trying to translate their own megadungeons for a wider audience. Like, it's a finished product, how hard could it be, right?
Going from my own travails, I would say it's because even for great DM's, 30% of their megadungeon is derivative or outright robbed and needs to be rewritten; 20% is silly in-jokes, goofy shit, or personal campaign-specific material that would be stupid/useless/offensive to people outside your group and needs to be rewritten; 40% is insane ramblings in notebooks that you just winged the rules for and the players went along with it (because FUN) and now you have no idea what those notes referred to, or are realizing the made-up rules would absolutely nuke a campaign in the hands of players less understanding than your own aaand you have to rewrite it; and a final 10% of stuff you ad-libbed at the table and never noted down and no one remembers exactly how it happened, but there's a totally unconnected map with some apocryphal symbols on it and you think maybe a Gate was involved? I dunno. Reeeeewrite it (it will never quite be the same or as awesome though).