1. It's poor architectural design. Good design should make it easy to get from place to place. Dungeons are designed to force you to go through choke-points.
2. There's often a secret passage for the "boss" that you won't find until you've reached the inner sanctum, but if you could find it early would save you a lot of trouble.
3. It's poor defensive strategy to split your defenses among a lot of separate areas when the goal is to protect a centralized treasure vault.
4. Carving out an underground tunnel system is a lot of work, making it larger than necessary is wasted effort.
All very good points. If your goal is to make your game more "realistic" to improve suspension of disbelief, and these sorts of things are things that you can't suspend your disbelief of, then yes, super logically laid out fortresses with easy ways to get straight to the end, and concentrated defenses where they will do the most good are the way to go.
And Shad does mention several times that he understands that dungeons are this way in order to facilitate game play. Good for him. And his idea towards the end of designing a rational, realistic fortress and letting players design their own plan of attack like a heist or caper movie plot can be fun, but I wouldn't want this all the time.
I've already given a pretty good reason why an underground labyrinth might logically exist a few years ago, so I'll let that post stand as a rebuttal to #4. If you don't want to click the link, I compare a map of the Mark Twain Cave, created by nature, to a typical dungeon layout.
For the idea that it's a poor defensive strategy to spread out your defenders, well, yes, maybe. But in most fantasy worlds, there will be wizards casting fireballs and ice storms and whatnot. Put ALL the monsters in one big room, and that handful of area affect spells the wizard has are suddenly a LOT more powerful. It's much better to get the twenty orcs, three ogres, two owlbears AND the blue dragon in one fireball than to have to decide to use it on only one of these groups of monsters.
If you were a BBEG, would you really want to put all your monsters/soldiers in one area where more than half could be wiped out by one fireball? In the real world, would you station all of your soldiers where they could be targeted by one artillery shell or guided missile? Of course not. Grouping your forces may be a strong defense against a conventional attack with swords, bows and spears, but not against area-effect firepower.
When it comes to the secret passage that allows quick access to the end, I think it's actually a good thing. If players grumble because they didn't find it early on, well, that's either because they didn't look for it, looked in the wrong place, or the dice just weren't on their side this time. Finding and taking advantage of that secret passage is good game play. And he mentions computer games like Skyrim don't allow you to find it at all. That's on the game designers, not a fault of the dungeon itself.
Finally, we come to the first point on architectural design. Now, the occasional dungeon with a logical architectural design can be a good thing. A nice change of pace. I was thinking about making a dragon's lair dungeon with a long wide corridor from the entrance straight to the dragon's den for the foolhardy adventurers to rush to their doom. Side passages would be for servants, food storage, etc. I wouldn't want every dungeon to be this way, though.
I think Shad is missing out on a few key concepts besides just game-play factors. And yes, that is probably the main reason for the multi-room, labyrinthine dungeon layout. Finding the treasure is supposed to be the challenge of the game. But there are a few concepts that Shad seems to believe are important that may not be, or at least aren't always important. And I think he hints at one of the biggest reasons for a dungeon to be the way it usually is, but doesn't quite make the leap to realize its importance.
First of all, Shad puts a premium on realism. Understandable, as that's the whole point of his YouTube channel. Do research on historical arms and armor, then point out how fiction/film/games get it wrong. For me, anyway, I think that too much realism is just as shattering to the fiction of the RPG session as too little. Making everything realistic is impossible. We need game mechanics to elide features of reality that are just too difficult or unwieldy to use in a game.
I remember getting turned off of the PS2 game Metal Gear Solid 3 because of its attempts at "realism" that made things LESS realistic. In that game, when you were wounded, instead of the elegant but ridiculously unrealistic method of eating food to cure your wounds (tried and true in many games), you had to go into your equipment management screens and treat the wound the way a field medic would. Clean it, anesthetize the immediate area, use antiseptic, stitch the wound closed, more antiseptic, and bandaging. Realistic, right? But you could be in the middle of the boss fight, pause the action, perform minor field surgery on yourself, and then restart time and the boss is right where you left him. That threw me enough to ruin my suspension of disbelief, and then the hassle of needing to complete five or six steps when in previous games I had only one to solve the same problem made the game unfun and I never finished it. (Pretty sure I've posted about this before here on the blog, sorry for the repeat.)
The point is, trying to become more realistic in one area made the game even less realistic in another area.So there needs to be a proper balance between realism and elegance of mechanics.
Secondly, Shad seems to be around 30-ish, so I'd guess he probably started RPGs in the 3E era, or maybe 2E AD&D/White Wolf era. He seems to take a lot of things that were popular back then as a given for game design. He mentions several times that to him, a "dungeon" should be a villain's base and why would a villain want to have to go through the ogre's chamber and around the flaming flying dagger trap every time he wants to nip out for a coffee or a pizza?
My question for Shad is, why do you assume that every dungeon is some master villain's lair? Some dungeons are, yes. And they would be better off to at least conform somewhat to Shad's cries for realism in dungeon layout. But not every dungeon is a lair. Some are just caverns. Some are tombs. Some are treasure vaults. And some...well, we'll get to that in a moment. Not every dungeon should have a BBEG lurking at the end. Not every dungeon needs to be a livable space. That's not written anywhere in any D&D book I've ever read. In fact, several of them are explicit that most dungeons are NOT.
Finally, here's the part where Shad almost gets it, but not quite. He mentions, around the 12:15 mark, that illogical dungeons are almost set up as if it were designed as a challenge. That someone wants the adventurers to get the treasure, but only if they prove their worth. But why would someone do that? It's illogical! [Setting aside the fact that in the real world, that's exactly what DMs are doing!]
Enter the realm of the dungeon as mythic underground. Modern fantasy obviously had its roots in mythology. Tolkien, Anderson, Howard, Moorecock, Dunsany, Morris... Lots of early fantasy writers drew on mythology and transformed it. There are plenty of blogs out there about using the dungeon as a sort of otherworldly zone where mortals can challenge themselves and prove their heroic worth. And yes, it can be seen as a handwave to explain away things like why there are no orc babies or what do the dragons eat when there are no adventurers to snack on. But it also gives the game a sort of resonance and weight that can be very impressive and immersive for players.
If the dungeon is a mythical underworld, rather than part of the normal, real, rational world, then Shad's idea is exactly right. The dungeons exist, put there by the gods or the Cosmic Forces of Law and Chaos, or whatever explicitly as a challenge to would be heroes. Are you strong enough to overcome these monsters? Clever enough to avoid falling victim to the traps? Wise enough to navigate the maze of passages without depleting your resources? If so, then congratulations! You win the treasure!
As Shad mentions, any sane evil overlord would want to protect their wealth, not offer it up as a challenge for the worthy. But we've already established that Shad's preconception of a dungeon as primarily a BBEG lair is already clouding his judgment on this issue, and that's why he fails to make the cognitive leap to the mythic underworld concept.
If the dungeon is the setting for a Campbellian hero-journey, then of course it should be set out this way. Every choice of pathways is a lady-or-tiger dilemma. Every encounter is there to challenge one or more aspects of your character. And yes, it is purposefully created to be difficult but not impossible to succeed.
If, like Shad posits, all of your dungeons follow the strictures of good architecture, all are bases for some BBEG or another, and all are defended in the most logical way, you can never achieve this sort of mythic resonance in your sessions.