If the catastrophe is not global then the knowledge of making magic items is not lost as nations not affected by the catastrophe keep this knowledge.
Disaster, War and Plague are catastrophes, Cultural upheaval, Religious opposition and materials getting expensive are local factors which can not wipe out knowledge on a more than local scale unless that knowledge is useless anyway (not the case with magic items).
Disaster, war and plagues ARE catastrophes, but not:
...gigantic catastrophe(s), much more destructive than anything humankind experienced in the real world.
Knowledge is not uniformly and universally distributed. Never has been. Just because a nation has an item, it does not follow that they know how to make said item. Even in the Information Age, nations have managed to keep technological secrets to themselves.
And if a local event such as listed above struck such a nation, those secrets could be lost for decades. Or even forever. Sure, someone might discover
some way to do what they did, but it won't necessarily be the same as the way their predecessors did it.
How long did Asian nations keep gunpowder from spreading West? What if, before it did, the Black Death had wiped out those who knew its secrets? What if gunpowder had been declared religious anathema? An affront to the gods?
Neither of those happened, but could have- the number of alchemists keeping the working formulae was not huge, and some of that knowledge was kept compartmentalized. And it would be hard to argue that the secret of gunpowder was "useless".
On to magic in particular: what if Spell X requires the eye of a particular bird? Said bird is tasty, slow moving, and has no fear of humans, so is hunted to extinction. Spell X is lost unless someone can find a substitute for that bird's eye. And even if a substitute IS found, the new component's nature may be different enough that the spell functions differently...
Someone upthread suggested that magical arms and/or armor might only be made from meteoric iron or "starmetal". Starmetal is pretty rare, its distribution is random, and is usually only found in very small amounts. Given the amount of water on the typical game world, most of it is probably sub urged beneath hundreds if not thousands of feet of water. Unless you find a big, terrestrial strike, it might take you YEARS to find enough starmetal to make a single sword.
Have we, for all out technology, figured out how ancient societies around the world built mega-structures with precision out of massive carved stone that wasn't locally sourced? That would seem to be useful to know, wouldn't it? Sure, we have some educated guesses that have been tested, but no theory thus far is without flaws. (AFAIK.)