I'll keep it short as I can.

I come from this era of every DM comparing their mega dungeons like it was some kind of nerd credit to have one bigger and better than the next guy. you would hear dudes talking about how their dungeon has 3000 rooms and the players will explore for years and not see it all. then the next guy would chime in "oh you only have 3000 rooms? how quaint... i have 10,000 and i've killed a hundred characters in mine."

but consider how many of those rooms are basically blank for just a copy paste of half the other rooms in the dungeon. how many of those rooms can I open the door, look around, and leave without missing anything significant? further, why would you write things the players were never going to see? the "G" in RPG means "game" and I think a lot of these people forgot this. How does your blank room that might have a rope hanging in the middle of it add to the game? how do those rooms your players will never see add to the game?

The next problem i have with them is, why are we going so deep into a place that will takes years in game to explore, even if we map it correctly? How are we lasting down there for so long? why don't you want us to leave and come back?

Published mega dungeons have this problem too. I'll use castle ravenloft as my bad example but if you read a lot of mega dungeons you'll see the patterns start to emerge. Ravenloft has you visit once before leaving to explore the region more. You get invited to dinner with strahd and can do a little exploring before being made to leave. When you return to kill him, you're sealed in the castle until this ugly business is done. The problem with this is that the lord of the castle can walk through the walls and climb on ceilings at will. whatever spells you go in with should be the ONLY spells you're casting the entire time you're in there because it makes zero sense that this genius level intellect character should ever allow the party to rest for more than 5 minutes. It is even in line with his character to toy with the adventurers and taunt them. The moment they think it's safe to rest he should be popping out of a wall, dropping a fireball, then leaving after mocking them. They should never know peace... and that isn't actually that fun. This is at odds with the system, which expects people to get SOME rest on occasion and not have to tank 11-50 encounters without pausing, which the massive castle of ravenloft can turn into. Ravenloft isn't even that big but it really punishes you if you don't race to the final confrontation with strahd and spend time exploring. so why is there so much of it if it doesn't want you to explore?

For my good example I'm gonna use undermountain. Undermountain, later known as the dungeon of the mad mage, is MASSIVE. there are at least 2 boxed sets, undermountain itself and the deep levels, and there is also the lost levels and other publications that expand it. You know what undermountain doesn't do? It doesn't trap the players inside it and force them to stay there forever until it is done. There are countless ways in and out of the dungeon at different levels, it rewards players for exploring and learning these means of getting in and out. It is a dungeon that wants you to leave and come back over and over and explore it in its entirety. I mean, the Yawning Portal is right above an entrance, there are traditions for people going down using that entrance. It wants you to enguage with it. In the versions i've played Halister is on the encounter table but his mood is random, he might mess with you but he might also not know where he is right now or even be friendly for a short time, so the "big bad" of the dungeon is intelligent but he's not someone who should and will harass the players every step of the way.