This really goes both ways. Why don't they do learning on pre-formed groups? It's also a server issue. Even for farm parties, small pop servers have a very hard time forming pre-made. And I guess same could be said for forming pre-formed learning parties.
The big problem is not enough people even attempt to try it until it gets nerfed. Thus actually learning (or never learning until they actually get carried) the fights way way way after they were implemented into the game, which causes this huge rift of every 1 person to know the fight, being grouped with 2 or 3 who don't.
Once it becomes obsolete content, I figure the kill lockdown should also be unlocked. But theres gonna be a ton of people who can easily clear FCoB, and their weapon will not drop while its under a weekly lockout, and sadly will have to DF it with and deal with all the blinds.
And i'm not sure if there's an official statement anywhere, but judging how they stager releasing content in DF after making it pre-formed party only, I do think they designed it so that things appear in DF after they are considered easily farmable.
I use DF expecting to get clears, not having to write a guide.
The only exception is a legitimately new player starting the game and you can usually tell by their gear.