Meh. Simply don't care.
Why can those monks break spells? Because they are Spellbreaker Monks. Can you learn to do it? Nope.
I have zero interest in that level of world building, either as a player or a DM.
But, I was specifically talking about in published books - as in what the game designers do, since that is something that's been brought up multiple times and even
@Micah Sweet specifically responded to with "opaque" game design.
It's not opaque at all. It's perfectly easy to understand. The monster can do X. Why? Who cares? How? Doesn't matter. Why can dragons fly? How do they breathe fire? Again, that's just what dragons do.
And the second that WotC actually does come out with an explanation, you get the typical response that they are being "disrespectful" to canon or the original creators, or they've ignored some 30 year old Dragon magazine article or a thousand other things that people will bitch about whenever WotC actually does come out with any sort of explanation. Endlessly and over and over again.
I'd much, much prefer they just keep silent. Then all people can do is complain that there is no lore. That's an easy thing to fix, as you so rightly put it. Write your own that works for your table if you need to.
I'm just so unbelievably tired of listening to the endless bitching about every single thing that WotC does that I would much, much rather they stay silent and not feed the trolls.