Let's talk about DM Kayfabe, Tucker's Kobolds, and the dire elephant in the room:
DUNGEONS & DRAGONS HAS A LETHALITY PROBLEM
You know what I'm talking about; goblin hit-and-run tactics, midnight drow ambushes, dragon strafing runs; an awful lot of DnD monsters are miserable to fight if they're played with anything resembling intelligence. At the same time, there's a long tradition of DMs being encouraged to endow their monsters (especially the ones with high Intelligence scores) with the political nous of Michiavelli and the strategic and tactical brilliance of Sun Tzu. How, then, has this not resulted in the median experience of playing DnD consisting of a series of one-sided OC massacres? I'll get to that.
But first I need to explain what makes DnD (specifically modern, post-4th-edition DnD) so deadly.
1. It's essentially a narrative skirmish wargame without Down Mechanics
By "Down Mechanics" I mean things like Warhammer Fantasy RP's notorious Critical Damage tables, Ultraviolet Grasslands' Oracle of Death, or Ironsworn's Face Death move; mechanics that serve to mitigate the chance of actually losing a character without cheapening the impact of, for example, Gertrud the Ratcatcher getting slashed across the face by a blood-mad sellsword. Generally, these are much less deadly than they initially appear.
For example, WFRP 2e's famously gory tables are, it's true, filled with limbs being turned into "bloody ruins" or "dangling masses of bloody meat," but they're also gated behind 1. losing all your hit points; 2. the initial Critical Value Table, which uses the individual damage of each attack suffered at 0 hp to determine the actual range of critical damage available (a tough and well-armoured character can actually survive a lot of attacks at 0 hp for this reason); 3. Fate Points, which can be permanently spent to miraculously escape death and dismemberment (the cut looked worse than it was, they were only knocked unconscious, a convenient tree broke their fall). This makes the Grim and Perilous Old World somewhat less deadly, which is great if you like character development!
Dungeons and Dragons 5th Edition doesn't have those, outside of some vague optional rules for permanent injury hidden deep in the Dungeon Master's Guide, between the vague optional rules for metacurrency and the rules for designing monsters that appear to have been purposely written wrong as a joke. Instead, when a Player Character falls to 0 hp, they fall unconscious and start bleeding to death, which is a problem because:
2. They made healing bad so people wouldn't be pressured to play healers
Healing is bad in 5th ed. Like, really bad. Like, the only ways to be an effective healer are either 1. busted shit like the Twilight Cleric (which can give everyone inside a bubble a steady drip of temporary hp every turn, which is really fucking good (just take my word on it)) and Healing Spirit (before they changed it to only heal a few times before vanishing) and; 2. "Yo-Yo Healing", (AKA "Chumbawumba Healing" or "Python Healing") where you cast Healing Word on an ally who's bleeding out on the cold dirt floor with 0 hp, at which point they leap to their feat, cry "'Tis but a scratch!" and get stuck in once more until someone hits them, at which point they are, once more, bleeding out on the cold dirt floor until you cast Healing Word on them and they get back up again and so on and so on. This is both really fucking stupid and a problem because:
3. It's very hard to kill Player Characters unless you want to kill them
This is not a tautology. Once 5th ed. PCs get to around 5th level, they start becoming increasingly impervious to incidental harm. Where once Bob the Fighter getting fatally shanked by goblins would have lead to - at best - a hasty retreat to the nearest temple to have him Raised, now Brother Bactine just rubs a 300 gp diamond on the corpse and Revivifies Bob on the spot.
But if the DM wants to kill the PCs stone dead, it's simplicity itself to prioritise targets and utilise the right spells to add another notch to the TPK tally. Which is bad because people can, in fact, tell when they're being played against rather than with, and you almost have to play against to kill PCs in 5th ed. Classic examples of this are Counterspelling Revivify, hitting the <100hp Wizard with Power Word Kill, and opening a bossfight by Feebleminding the Cleric. This is a problem because:
4. A lot of monsters are incredibly dangerous if the DM doesn't replace their brains with tofu
The classic case of this is of course Tucker's Kobolds (though in that case the players appeared to have had their brains replaced with tofu), but a much more recent and much less contrived example is the opening of the 5th ed. Starter Set, Lost Mine of Phandelver, where four goblins ambush the PCs on a road through a wood. Goblins, it's important to note, are not only sneaky, they can also hide without using an action, letting them, for example, attack and conceal themselves on the same turn. Now, if the goblins were allowed to use their stated intelligence of 10 (i.e. average intelligence for someone without a lump of tofu where their brain should be) they would take advantage of the terrain and their innate skill at stealth to pincushion the PCs with arrows from the trees without ever coming within reach of their swords. But since this would almost certainly lead to a Total Party Kill, instead two of the goblins "rush forward and make melee attacks" while the remaining pair of goblins "stand 30 feet away from the part and make ranged attacks," because they have tofu for brains.
For a much higher-level version of this, let's look at the grand final battle of Rise Of Tiamat, the first big high-level campaign for 5th ed. After fighting their way through a temple in the caldera of an extinct volcano, the PCs storm into Tiamat's Temple, an Abyssal nightmare brought into the mortal world by unspeakable sacrifices, the centre of a hideous ritual to bring the Queen of Dragons into the waking world and bring about an era of endless tyranny blah blah blah it's a big deal. Anyway, there are nine nameless Red Wizards performing the summoning rite led by Rath Modar, a big-deal Red Wizard. Also there's a guy who's really just hanging around not doing much.
If at least five Red Wizards don't spend their action performing the ritual two rounds in a row (remember this), the ritual will be disrupted. Now, if Rath Modar and his backup dancers don't have heads full of tofu and are instead played according to their character blurbs and stats, what's going to happen is the PCs are going to barge into the ritual chamber and cry "Have at you, foul varlet!" to which Rath Modar will reply with an ice pun and casting Globe of Invulnerability (which, for those of you who don't care about high-level DnD (and who does?) creates a 20-foot-wide immaterial globe that makes everyone inside it invulnerable to spells below 5th level, even if they're cast with a higher-level spell slot). Then the Red Wizards and that one guy will pile into the fuck-your-magic-zone and blast the PCs with Cones of Cold, which has pretty good odds of killing the entire player party unless they 1. had the foresight to cast Globe of Invulnerability with a 7th-level spell slot or 2. cast Antimagic Field (which is to say, if they didn't pick the right spells, they're fucked). And the Red Wizards can go back to doing the ritual on the next round, and on the round after that they can all Fireball the PCs.
All these things together mean that if DMs don't play monsters like they've got tofu for brains, PCs will drop like flies and stay dropped, which is bad because modern DnD is primarily a game about creating OCs and thinking about how cool they could be, which is hard to do when all four members of The Powerful Five (Princess Raven Way, Fux Hughly, Captain Caricature, and Gh'ei Bey't) are lying on the the ground in pools of their own blood.
Fortunately, Modern DMs have developed a couple of techniques for avoiding this unfortunate situation.
1. Throwing fights
This encompasses but isn't limited to playing monsters like they've had their brains replaced with tofu; it also covers miraculous rescues and just straight-up fudging the numbers (*rolls behind screen* "phew, he just misses you."). The other players will inevitably begin to catch on, which neccessitates:
2. DM Kayfabe
Kayfabe is pro-wrestling lingo meaning "Acting," specifically acting as though you really do put on tiny shiny shorts to go hit and be hit with a steel chair in deadly serious martial arts matches. In this context, DM Kayfabe is when the DM acts as though they really are trying to totally destroy your precious Player Characters in deadly serious elfgame combat when in actual fact they are having difficulty rolling the dice because of the pillows they've strapped to their fists. This is at least half the reason that there's so much fucking Evil Killer Dungeon Master merchandise - you know, shirts with Beware The Smiling Dungeon Master on and fucking, dice with skulls on them and shit (the other half is mostly actual shit DMs who think killing PCs makes them hot shit and not essentially a bored, vindictive Sims player). This would be all well and good except for the Suplex Problem.
The Suplex Problem is that, much like the popular wrestling maneuvre, it's possible to perform Killer Dungeonmastery unassisted, which often leads to hurt feelings, shredded character sheets, and broken-up DnD groups. In fact, it's really easy, often starting when some dumb teenage DM looks at a premade villain's statblock and spell-list and says "Well, this doesn't make any sense. Why would a brilliant wizard like Mitch the Malevolent pick Cone of Cold for his 5th-level spell when he could pick Wall of Force?" Because as soon as you start endowing your antagonistic NPCs with brains not made of tofu, their effective deadliness ratchets up massively (remember, tactics as simple as "casting their best blasting spell from maximum effective range" and "shooting the healer first" are enough to seriously endanger PC parties).
Unfortunately, all these problems are basically fundamental to post-4th-edition DnD, and "fixing" them would require rebuilding the game almost from the ground up.