It’s how real-world alchemists did things
Which is likely something that Gygax—and Vance, assuming Vance’s novels also including writing spells down—would have known.
There are famous alchemists. Plato is arguably the father of “Western” alchemy, and he’s one of the most-published authors of all time. Jābir ibn Ḥayyān is the first person in recorded history to write down many different chemical properties and processes. In Europe, alchemists sometimes rose to prominence in the courts of kings and queens—John Dee, Nicholas Flamel, and the like. Even Isaac Newton took some interest (though he took much more interest in astrology).
But most alchemists didn’t publish and become famous. Most alchemists, it seems, didn’t want to. They invented arcane writing styles all their own to encode their processes—and jealously guarded those secrets. Many truly believed that they were working on achieving grandiose supernatural power—and didn’t want to share. Many, even, felt strongly that deciphering these codes they used was a security measure, not for themselves, but for the world. That only those who proved their wisdom by figuring out their writing could be trusted with the knowledge contained therein. Some of those codes, therefore, were meant to be cracked—but only by the “wise,” who were presumed to be trustworthy.
And even the better-known codes are still extremely arcane to us. Take a look at the Unicode page for alchemical symbols, for example:
||Alchemical Symbol For Quintessence
||Alchemical Symbol For Air
||Alchemical Symbol For Fire
||Alchemical Symbol For Earth
||Alchemical Symbol For Water
||Alchemical Symbol For Aqua Vitae
||Alchemical Symbol For Vinegar-3
||Alchemical Symbol For Salt
||Alchemical Symbol For Vitriol-2
||Alchemical Symbol For Dissolve-2
||Alchemical Symbol For Half Ounce
You’d find entire books written in these symbols, with minimal actual language around them. And worse, you’d find books written using the author’s own personal set of symbols, and maybe their page layout and organization is encoding more information not found in the symbols themselves, and so on.
I mean, for that matter, you can even look at modern chemical equations and can easily imagine how impossible they can be to understand: even the straightforward “2H2(g) + O2(g) → 2H2O(ℓ)” should immediately raise eyebrows for anyone who doesn’t remember chemistry class, and that’s a really famous one. Chemists standardize things heavily so that it can be taught in chemistry class, but that’s a relatively modern effort, pushed by modern sensibilities and enabled by modern technology.
But without a press, without regular international communication, with information that can literally burn down buildings or gate in demons? Yes, it is entirely plausible that wizards all come up with their own system. It’s certainly what the people who were trying to become wizards here did.
Also, just to bring this back to the lore a bit more...
One of the things with Vancian magic is that the level of memorization involved in casting a spell is absurd. It’s way, way beyond anything you are familiar with—in Vance’s works, if I’m not mistaken, the mere ability to learn magic was itself a magic ability, impossible for a regular human to perform.
There is much, much more information to encode than you’d expect, apparently, in writing down a spell. Moreover, one has to encode things that are extremely difficult to describe—precise movements and utterances, yes, but also magical attunement and mental patterns and so on. And that encoding has to resonate with your own magical ability to memorize that spell so thoroughly that the act of keeping it in your mind is magic itself. A prepared spell is an actual, existing “thing” in the world of D&D. It’s not physical, of course, but it is tangible, for those with the right senses. It can even be stolen, in certain cases. And once used, it’s entirely erased—no matter how thoroughly you memorized it (and it had to be very thoroughly), and no matter how many times you’ve done it before, the actual act of casting a spell is enough to scour that magical thought-construct that was your prepared, memorized spell, entirely from your mind.
Which is to say that your system of encoding has to interact with your own personal magic. Other people’s encodings don’t necessarily work for you—or yours for them. You can, with enough effort, decipher someone else’s encoding well enough to get it into your head, to prepare the spell. But replicating their style, particularly with a spell you have never seen them encode? There’s no hope. So you can’t copy their encoding perfectly anyway, even if you wanted to.
Tangent about alchemy
It’s important to bear in mind that “chemistry” didn’t exist at the time. “Alchemy” was the name of the serious pursuit of trying to learn more about what makes up the materials around us and how they can be manipulated. Plato... honestly, Plato wasn’t right about much. But Jābir is the first recorded discoverer of many chemical substances and techniques. And he wasn’t alone in making real discoveries. But it was still also, ya know, “alchemy,” as we think of it—turn lead into gold, achieve immortal life, etc. How much a given alchemist focused on those things varied widely, of course, but the split between material and spiritual concerns that we have in the modern world didn’t exist then. They were both equally “alchemy,” and both equally serious.