If it's so easy, then why aren't YOU doing it? You could write it all up yourself and post it to DMs Guild and get people to buy it from you.It could be structured something this.
Old Empires Region
Brief regional over view
Nations:
Mulhorand:
Basic Overview of Mulhorand and it's themes.
Capital City (population, major imports/exports), a less detailed list of other important cities/locations in Mulhorand for adventurers.
Religion/Government/Demographics.
After the Sundering detailing how the Sundering changed things.
Plot hooks
Unther
Capital City (population, major imports/exports), a less detailed list of other important cities/locations in Mulhorand for adventurers.
Religion/Government/Demographics.
After the Sundering detailing how the Sundering changed things.
Plot hook
Chessenta
Capital City (population, major imports/exports), a less detailed list of other important cities/locations in Mulhorand for adventurers.
Religion/Government/Demographics.
After the Sundering detailing how the Sundering changed things.
Plot hook
Tymanther
Capital City (population, major imports/exports), a less detailed list of other important cities/locations in Mulhorand for adventurers.
Religion/Government/Demographics.
After the Sundering detailing how the Sundering changed things.
Plot hook
Akanul
Capital City (population, major imports/exports), a less detailed list of other important cities/locations in Mulhorand for adventurers.
Religion/Government/Demographics. Side table for Mulhorand Pantheon listing it's Gods and key details for their clerics (worshippers).
After the Sundering detailing how the Sundering changed things.
Plot hook
Minor powers.
Then rinse and repeat with the Cold Lands, Shining South, Lands of Intrigue, Heartlands, Shaar, Chult, Lake of Steam, Sea of Fallen Stars, Turmish, Halruua, Unapproachable East, ect...
It's actually pretty easy, most of the work is figuring out how has this region changed by the Sundering a the events thay followed it, and adding fresh new plot hooks.
Later in the book they can put whatever Player options, DM rules, and Monsters they want in it, even very optionally an intro adventure, although in FRs case with all the APs and other adventures, I'd focus on the rest instead.
Oh right... because that wouldn't be real. You need real Forgotten Realms. And the only way the story of the Forgotten Realms is real is if people who work for the brand that controls the Realms writes something that the brand then publishes. If that happens, then what was written was real Forgotten Realms. Anything else that anyone else does isn't real, and thus doesn't matter.
But as has been said time and time again... the brand that publishes the Realms has stopped caring what is and isn't real anymore. They aren't concerned with canon. The need for a long, drawn-out, ridiculously overwritten timeline and history of hundreds of locations over dozens of millennia is gone. The people who control the brand just... don't... care. They are off the canon treadmill. Because it's a waste of time, a waste of money, and a waste of energy. And it serves no purpose except to make a few obsessive-compulsive fantasy history buffs happy. But the people who control the brand aren't here for you anymore. They're now here for the other 99% of the population who only care about the Forgotten Realms for what it can do for their game. Not for the history books, but for their game.
If it's useful for everyone's game at the table, like say for an adventure that people can actually sit down and play... then sure they'll fill in some details like for the Sword Coast or Chult. But they aren't going to write setting details just for the sake of it because some people have this ridiculous need for their history book to continually be full. If you want your history book full... then write it yourself and finally accept the fact that the stuff you write is real for your Forgotten Realms campaign game. It is now just as real as anything Christopher Perkins may write.