I think I had a minor epiphany. The issue is abstraction vs concrete presentation in a statblock.
Take the NPC acolyte. Its supposed to represent a friendly village priest, a servant of a dark god, a drow warrior, an enlightened mystic, and probably a dozen other characters represented with minor divine magic access. Can the stat block represent them all? Well, yes but no. Yes is the broadest sense: they give you an AC, HP, attacks, and spells that pose a specific challenge level, but on a micro level fails to account for species, specific god worshipped, training, role, etc. If the village priest of Pelor teamed up with the enlightened mystic from the mountains and fought a tiefling servant of Nerull and his drow consort, they'd all be doing the same fricken thing to each other despite clearly representing four different people with wildly different backgrounds and abilities.
Because D&D doesn't want you to think of the stat block as a concrete list of what the NPC can or can't do. It represents a set of stats that can be used to represent them, but it's not the all-inclusive list of features we grew used to. Its abstract, designed to be explained any way that you see fit. And its done that way to allow them to be used and reused for a variety of things the DM would want. When you look at it from that lens, things start to make sense. The lack of species traits for NPCs. The lack of custom monster creation. The level of abstraction in monster stat-blocks. Its because WotC wants you to create the fiction first, the grab a stat block that "looks close enough" and use that to represent it if needed. The creature ISN'T the stat block. The stat block represents the creature if its necessary.
I will admit, its a very different look at D&D, but its not an inconsistent one. WotC doesn't care if you use the hobgoblin warrior to represent a human soldier, an orc mercenary, or even an undead knight. All you do is make a few cosmetic changes (such as the ones outlined in the DMG) and use the stat block. In that light, there is no reason to justify the longsword damage because that stat block can represent an multitude of different heavily armed warriors, hobgoblin and longsword are just the default names for them.
Ladies and gentlemen, disassociated mechanics is back. But only for NPCs. The complaint about 4e usually came down to how PCs had disasociated abilities (powers) that didn't make sense in the flow of the fiction. But nobody really complained about that for NPCs and monsters. Mostly, because those are defined by their opposition to PCs and thus only matter for the purposes of their role as combantants, helpers, or whatever. The old MM did this to a degree, and the new one has embraced it more fully.
PCs use concrete rules, mostly. Monsters use abstract rules. Each does it differently based on their role in the game. To them, the game rules do not simulate reality. They said as much in the DMG. The rules, the stat blocks, they aren't designed to simulate the world, only to give characters a way to interact with it. But its consistent through the game when you look for it. "Don't let your players argue physics to get out of a game rule." "Four different NPCs can all use the same stat block". "Don't allow PCs to game the economy". "You can reskin a hobgoblin to be anything you want." "Your PC can take any background and change the lore so that your noble actually represents a wealthy gambler."
Yes, that is going to be a rude shock for people who want very concrete rules. And that is a change (or at least a certainly an evolution) from older D&D (specifically 3e) where concrete rules applied across the whole game equally. And Its not going to be for everyone.
