What I mean is that even though these maps show a lot of rooms, presumably not every room was of equal importance or interest. Look at Gygax's level-stocking advice from OD&D vol. 3:
In laying out your dungeons keep in mind that downward (and upward) mobility is desirable, for players will not find a game enjoyable which confines them too much. On the other hand unusual areas and rich treasures should be relatively difficult to locate, and access must be limited. The layout of a level will affect the route most often followed by players. Observation of the most frequently used passages and explored rooms will guide the referee in preparation of successive levels, which, of course, should be progressively more dangerous and difficult.
and
As a general rule there will be far more uninhabited space on a level than there will be space occupied by monsters, human or otherwise. The determination of just where monsters should be placed, and whether or not they will be guarding treasure, and how much of the latter if they are guarding something, can become burdensome if faced with several levels to do at one time. It is a good idea to thoughtfully place several of the most important treasures, with or without monstrous guardians, and then switch to random determination for the balance of the level. Naturally, the more important treasures will consist of various magical items and large amounts of wealth in the form of gems and jewelry. Once these have been secreted in out-of-the-way locations, a random distribution using a six-sided die can be made as follows...
Taken together, these quotes say to me that the majority of each level will be empty rooms and/or randomly-assigned "filler" encounters, that only a relatively small fraction of each level will be the truly interesting/rewarding areas, and that those areas will be remote and/or hard to locate/access. So for skilled players dungeon expeditions will consist of searching out and going to these "areas of interest" while spending as little time and as few resources as possible dealing with all the "filler" stuff in-between. So yeah, "Door #287" is there, but only inferior players will waste a lot of time and resources on it, because their more skilled brethren will be in (or at least on their way to/from) the interesting/unique/rewarding areas.
I've described this idea elsewhere with the shorthand of "dungeon-as-wilderness" -- just like a wilderness-based campaign has various areas of interest (lairs and mini-dungeons) secreted away in the midst of an expansive (and generally not all that interesting) wilderness full of wandering monsters which players will generally seek to cross as quickly and uneventfully as possible, "campaign-dungeons" (i.e. so-called megadungeons) are the same way -- small areas of interest (rooms and sub-levels) surrounded by and hidden amongst lots of generic/filler areas which smart players will seek to avoid as much as possible. Instead of traversing the Devil's Woods and Howling Hills to get to the Tower of Woe, the characters will be traversing Dungeon Level 4 to get to the Chapel of Dalt, but the dynamic of play is essentially the same. Just like characters in the former can choose to loiter in the Woods going from hex to hex having random encounters, but doing so is generally a boring waste of time and resources and a distraction from the stuff that's actually interesting (the tower), characters in the latter can loiter on Dungeon Level 4, searching every door for traps and every room for secret doors, making sure their map is completely accurate and that they've rousted every roomful of giant rats and spiders, with the same result -- they will have wasted all their time and resources on boring/rote/routine stuff instead of what's actually interesting and unique (and fun) -- the special areas.
In a module-sized dungeon where there are likely only 30 or 40 rooms in the entire dungeon and every one of them has been designed to have something interesting and unique and potentially challenging and rewarding about it, it makes sense to be thorough in exploration, to visit every room, kill all the monsters, find all the secret doors, recover all the treasure, etc. But in a "campaign-dungeon" where there are likely hundreds, or even thousands, of rooms and 80% or so of them by design are at best empty and uninteresting and at worst deliberate time and resource wasters, a different approach is required as a matter of simple expediency -- you'll never be able to thoroughly explore the whole thing, so it only makes sense to spend as little time and resources as possible on the filler and focus as much as possible on the "good stuff."
And this, I think, is what Gygax is really talking about with his emphasis on setting specific objectives in the "successful dungeoneering" essay in the 1E PH -- that in a dungeon expedition players shouldn't be satisfied just going randomly (or even systematically) from filler room to filler room having what are effectively random encounters, they should have an objective of first finding and then exploring the interesting/unique/rewarding special areas. I suspect this grew out of frustration with running Greyhawk Castle for players who weren't part of the regular Campaign at conventions -- he knew there was cool stuff secreted all over the dungeon, but these folks weren't even looking for it because they were too busy going from one generic 20x20 room to the next rousting random groups of kobolds and centipedes. (That's what is so fascinating (at least to me) about that essay -- most of the advice he gives really isn't all that applicable to D&D the way most people play it (with a small player group and stable party, discrete module-sized "adventures," etc.); it's much more a guide to successful dungeoneering
in Greyhawk Castle -- a campaign with a large and fluid player-base, a huge central campaign-dungeon, and near-total player freedom of action.)