I want just enough explanation to tell me why, if a player has their character make armour out of that dragon's hide and scales, the character's AC bonus from that armour won't be as high as was the dragon's. That way, when the player (justifiably) asks about this, I have a ready-made (and backed-by-RAW) answer.
Otherwise, when the player raises this question, I'm left twisting in the wind.
This is super easy to come up for an explanation, though. You could say the AC is partly magical, and that magic begins to fade when the dragon dies. Or the process of creating scale mail armor from dragon scales weakens them and causes some scales to loosen/fall off. Or that the lower AC is because on a Dragon, the scales cover almost the entire body, while humanoid scale mail doesn't.
And if you can't come up with an explanation on the fly . . . is that such a big deal? Is the simulation of D&D so fragile that your fun is ruined when the DM tells you that a feature exists for game balance purposes and it would ruin game balance to let PCs get all the features monsters have? Immersion isn't the end all be all of the game. Fun is. Fun can be ruined by unbalanced mechanical features. Is it better to preserve immersion and grant the players a set of +3 Scale Mail that gives you immunity to fire damage for killing a Wyrmling Dragon when they're at level 3, or is it better to preserve game balance and still give them the opportunity to make a nice magic item but not give it the full potential of the Dragon's version? Or if you make the mistake of giving an overpowered magic item, do you maintain immersion and railroad the party into losing it, choose to just deal with it and let them keep the OP magic item, or do you explain that it's causing problems for game balance and nerf it?
Does every monster in every bestiary in every 5e book need to provide an explanation for the AC, hit points, damage, ability scores, and other monster features? Or can a Dragon just have 19 AC because they're a dragon and need to be hard to hit. Does every monster need rules for why the PC equivalent of a feature is weaker than the monster's if they choose to make a magic item out of them? Or can the rules leave that up to the DM and not waste space on pages of repetitive and bland lore justifications? Why can't game features be designed around making the game balanced and not be constrained by requiring exhaustive lore justifications for every minute mechanic or perceived discrepancy?
The answer for the incessant "why? why? why?" of mechanical features or aspects of the game is almost always "A wizard did it". Why do owlbears exist? A wizard did it. Why do orcs exist? A god created them. Why can't I cast
Shield of Faith and
Guiding Bolt on the same turn if I have enough spell slots? Magic has rules, and those rules say
Guiding Bolt isn't a cantrip. Why does that giant's greatclub deal 3d8 thunder extra damage but lose that when they die? Magic. Why are scales more protective on living dragons than as scale mail? Magic. Why can that Avatar of Vecna cast Meteor Swarm 3 times a day? Even level 20 Wizards can't do that! It's because he's a freaking god and the main villain gets god magic.
As a DM, it would just get exhausting to have to give expansive explanations for all minor details when the reason is for game balance and the lore justification will always be some flavor of "A wizard did it". It did get exhausting when I tried to do that when I first started DMing. Eventually I grew out of the notion that every mechanic needed a lore justification and decided game balance was more important. And it seems like WotC has learned that too, given the mechanical changes to recent monster stat blocks.
