Also, you don't need to know a Catch to use it. IIRC, Harry discovered Nicodemus's Catch by using it. And if I use a steel sword or a holy warhammer I'll likely satisfy all sorts of Catches by accident.
As I recall, Harry half figured it out, half guessed, before he made the attempt, so he didn't just completely discover it out of the blue.
And yes, wielding one type of item might let you find a handful of catches out by accident. But this isn't about accidents and serendipity. This is about deliberately, purposefully building a weapon to satisfy even those +1 rebate catches (which means it's something both rare
and very difficult to find out about), and if you're going to exploit a catch on purpose, it does pretty much require you to know what that catch is.
And there's nothing in the setting or the game preventing you from just saying "it's magic". Seriously, nothing.
Explicitly, no. But this is a game system and setting where magic has rules--things it can and cannot do, and how.
I don't see any mention of a wizard being able to just magic up the weakness to something without figuring out and gathering its components--Harry needed to capture sunlight in a hanky to exploit it against the Red Court, he can't just make his magic 'Holy' outside of using Soulfire, and he needed the inherited silver in his amulet to harm the Loup-Garou.
So no, the books don't explicitly say you can't justify satisfying a bunch of catches all at once with "it's magic." What they do have is a set of rules for what satisfies what catches, and several examples in the fiction as well as the rulebooks where even strong wizards have to research, collect, and prepare specific catches for specific creatures using the components of those catches.
The rebate value of the catch is explicitly based on how difficult it is to find and use the associated weakness, remember. The Loup Garoux is a terrifying, nigh-unstoppable monster instead of just a somewhat-difficult fight because it paid those extra refresh to have its weakness be something rare. The Black Court were so powerful because their weaknesses weren't widely known, and the publishing of one book with those weaknesses basically broke them.
Knowledge is power, and the whole Toughness/Catch rebate system is based on how difficult it is to find and exploit that knowledge. Giving a power which just gives the power without some serious justification flies in the face of that. There's a reason that the only canon power to do that only exists in three objects in the whole world and is backed up by an extremely powerful sponsor.
