I think much - if not nearly all - traditional fantasy at least implies a dark age with a post-Atlantean,"post-apocalyptic" backstory, where some of your best weapons and armor are wondrous artifacts of a fallen and lost technologically-advanced civilization. It's "after the Flood", after the fall of an empire, a rough and lawless New Age where people have just learned how to survive again, and the time has come for new Arthurian heroes to draw ancient swords from stone and carve out new kingdoms from the chaos.
It's why in the most generic sense, these settings tend to feature little pockets of isolated and imperiled civilization dotting a chaotic wilderness filled with crumbling ruins and forgotten dungeons, in need of adventurers to explore them, plunder them, vanquish the ghosts and monsters that haunt them, and restore order and hope to the world.
For example, Conan the Barbarian's Hyborean Age is literally post-Atlantean; Tolkien's Middle Earth from the LotR is set after the fall of Numenor; Clark Ashton Smith's Zothique is a far-future dying Earth near its end following the rise and fall of countless empires; Lovecraft's Cthulhu Mythos is a never-ending cycle of rising and falling civilizations of "Old Ones" in which human beings are only the most recent - and hardly the most important - participants, whose doom is already slated for their replacement with a nobler and longer-lived civilization of body-snatchers possessing intelligent insects who will soon sort through our ruins in wonder; G.R.R. Martin's Westeros is built on the ruins of the Kingdoms of the Andals and the fall of Valyria with other cycles of deeper history lost to time and countless apocalyptic winters implied before that; Dungeons & Dragons tends to be a bit sloppy with world-building in some of its more generic settings, but fallen civilizations play a key plot point in Eberron and Dark Sun, among others....
In short, the weird anachronism soup makes sense in context of general fantasy, and in that context, freakishly ahistorical full plate proably SHOULD be rare, compared to the cruder styles of "barbaric" armor, or the best but still rough imitations of the lost arts of the previous age that dark-age smiths could manage.
And, this sort of armor becomes the stuff of cursed "Death Knights" because they are the shadows and ghosts of the old order, lingering long past their fall into the chaos following the old order that had slipped from their hands.
Whether the anime artists realize it or not, the armor they're imitating was originally meant to be the work of antedeluvian craftsmen working with magical armor technologies from civilizations far in advance of our world, whose like has vanished from the world, and might never be seen again.
It's perhaps appropriate if there's an ahistorical, futuristic, sci-fi foundation to such armor and weapons! They aren't meant to be the next logical step up from what the average warrior is wearing, but instead something many steps ahead of the current age is capable of producing, looted from the tombs and crypts of giants, crafted thousands of years before by the primordial wizards and extinct civilizations from a more advanced but ultimately doomed cycle of time.
Well, anyway, one can get a lot of mileage for rank-and-file fantasy armies from historical kits like those from The Baron's War, and from assorted dark ages and even ancient kits (along with some of the more pulp-friendly "imperial conquests" subjects, for example), and the Renaissance is surely going to be increasingly helpful as well. I think the Renaissance is where we should see most of the more ornate historical armor and weapons, and it's probably at least part of the visual reference for "futuristic" fantasy plate armor! But, it's probably most appropriate for a fantasy knights kit to go full-steam into weird and elaborate suits of armor: if armies of "good guys" had access to that stuff, there'd be no need for adventuring heroes, and really, the setting would be missing something without the implied post-apocalyptic element in which it's mostly only the restless ghosts of the past who have access to the stuff....
