This is factually incorrect. Delete your account.
I
hope it was apparent when I said something like this that I was being satirical, making fun of the typically combative approach to forum debating. It was in response to a claim in which any stance is a matter of opinion. (Maybe you are being facetious, too. I can't tell, but thought I should I mention it.)
You're the GM. You don't need an explanation. And Wizards are not tribbles. Wizards will not start showing up in your GM notes on towns or something if you don't offer an explanation. Wizard stat blocks will not spontaneously proliferate in your adventures. Just don't make them, and they won't be there!
Player: "Why are there so few wizards and spellcasters in the world?"
GM: "Good question. There are many theories, but nobody knows for sure."
You only have to explain metaphysics if you are going to allow the players to interact with those metaphysics. Otherwise... it is just the way the world is. The PCs have no way to discover the answer without yoru cooperation, so you don't need to provide it.
Ok, sure, you don't actually
need an explanation. You can leave it like spellbook copying: i.e. that it seems very odd, if not downright improbable, that spells are rare and hard to acquire, given RAW. I, for one, find that dissatisfying.
Another example would old school dungeons, in which you find monsters in various rooms, with no explanation for what they are doing there, or how they get their food, etc. You don't
need to explain that either, if you don't want to.
And while I agree that you don't necessarily have to tell your players the answer, especially if you don't actually have one, knowing the answer yourself can help make for a richer imaginary world.
As an aside, it is weird that folks don't get this: the key to making magic mysterious is to not explain stuff.
I agree that a good way to make magic mysterious is to not explain stuff! Which folks are you referring to that don't get this?
AND I find only a tenuous connection between that observation and the topic at hand. YMMV, but most of the possible explanations for wizards being rare that I came up with have nothing to do with the mysterious workings of magic itself.
Now, you might have very few wizards in your world, and "nobody seems to know why" if the players ask. But to me that smacks of "I have no idea; stop asking dumb questions" rather than "oooh, how mysterious!"