In 3 years nobody amongst the 8 billion humans on earth ever tried planting anything in the dungeon? really?
Shit like this is why i can't take this manga seriously.
They tried, is the manga format that makes it pass as weird, need to read this and the chapter after to make it flow, the dungeon resets itself for any damage received and anything built/brought from humans in it is destroyed by slimes the moment no one watches.
A big building would be destroyed in parts no one is watching, and keeping a eye on it 24/7 is not possible, even a surveillance room wouldn't be good enough because slimes would likely spawn right out of the camera frame then close on the building altogether.
The rest spot in one previous chapter was just a camp with several people around and small gear in use.
This story have some fallacies, but for now I don't really see big plot hole, maybe some dumb actions from our MCs, but they were office workers, not special ops.