Why is the Forgotten Realms named that? Like everyone who knows D&D has a pretty good understanding of it. Its the setting that 90% of pre-written adventures are set in, so why is this setting the one everyone apparently forgot about 

Maybe because it’s inhabitants forgot about their history.

In one of my campaigns, we met someone from really far in the past, and the land had been very changed since his time- all of the names were different and no one remembered that his empire had ever existed. This was a published module- although I’m not sure how much was dm improvisation.

 There are also lots of ancient ruins in the forgotten realms- hideouts of lost empires and organizations that no one remembers that have been retaken.

Not a maybe, though UndauntedDM's conjecture is one way to play in the FR - I do more exploring traces of lost pasts in my games a lot too; but there's definitely story behind the branding here:

Ed Greenwood, who created the Forgotten Realms as his game world, had in his lore the notion that at some point the Earth and the Toreil were closely related, but grew apart and the knowledge of Toreil largely forgotten to people of Earth, except for some folks recovering the legends and myths of that world through playing a TTRPG. So Forgotten Realms is a fiction predicated in the IRL world, not some notion of absent minded ahistorical consciousnesses in the game world.  No one in the Forgotten Realms setting, aside from maybe a groaner of a pun somewhere I'm not aware of*, has ever said they are from the Forgotten Realms or addresses any part of Faerun or Toreil as forgotten realms.

*Maybe they're saving the pun as a gag line in the movie.

Yeah seems right to me 

In support of what MidnightPlat posted:

Forgotten Realms is a fantasy world setting, described as a world of strange lands, dangerous creatures, and mighty deities, where magic and supernatural phenomena are quite real. The premise is that, long ago, planet Earth and the world of the Forgotten Realms were more closely connected. As time passed, the inhabitants of Earth had mostly forgotten about the existence of that other world – hence the name Forgotten Realms. The original Forgotten Realms logo, which was used until 2000, had small runic letters that read "Herein lie the lost lands" as an allusion to the connection between the two worlds.

(https://en.m.wikipedia.org/wiki/Forgotten_Realms) Dec 5, 2021

Even better, this is Ed Greenwood's own words on it:

https://www.enworld.org/threads/ed-greenwood-how-the-realms-began.666535/

While I don't think knowing the pantheon of game designers is a necessary thing for playing D&D, I do think if you're playing set in the Forgotten Realms, you should at least be aware that the setting's creator is still actively producing content and has provided a lot of his thinking behind the setting's design (FR is much bigger than Ed Greenwood, but I think between him and Salvatore you get the "gist" of FR).  Just like if you play in Krynn you should know who Hickman and Weiss are, or Eberron knowing who Keith Baker is, etc.  It helps to know that there was a person behind the spirit of the world you're playing in, at least I think so.  At the very least it can build you the hubris to think you can build a world of your own too, and their creations become more instructions than boundaries or parameters.

The Forgotten Realms aren't ... Cormyr and Amn and the Lords Alliance. The Forgotten Realms are the hyper advanced civilisations that predated the modern era, like the Netherese Empire, who lived in flying cities. The Forgotten Realms are the ones who built a world of wonders and made all of the magic items that are litering ruins for you to find. Wonder why you don't find people making new sentient swords or holy avengers or other artifact level weapons? Because the people  and the realms who made them are all dead and forgotten.

"To you" the Forgotten Realms are evocative of the Netherese ... and who else again?  I mean if it was just the Netherese, wouldn't it be the Forgotten Realm?  And they can't be all that forgotten, I mean people are always trying to dig their stuff up when there's a trace...

I kid.  Regardless, your take is poetic conjecture of a consumer I guess, maybe extemporeously your own waxing or maybe apocryphal lore.  There's nothing wrong with either, it gives games playing with that notion a bit of mystique to the game world they're engaging.  But if you want to have an answer as to what Forgotten Realms meant to the creator of the Forgotten Realms.  Ed Greenwood, who created the Forgotten Realms and called them such well before they were an official TSR product, is on the record as to saying way.  I'm sure you'd appreciate that there's a history of textual production, and it's probably best not confuse the materiality circumstances of that production, including branding, with presumptions that there was a deeper lore angle at inception than there was.   

But yeah, when the grey boxed set came out, people who were playing "in the Forgotten Realms" were very much thinking they were playing in Cormyr or Waterdeep or Sembia.  I mean all those gazetteers to the Forgotten Realms published ... few if any in the first run touching upon pre Dale Reckoning civilizations.

If I recall, the Dungeons & Dragons animated series dug into the whole issue a little as those characters “fell through” a (and I”ll borrow from King for lack of a better term) “thinny” into the FR.

Quote from MidnightPlat >>

"To you" the Forgotten Realms are evocative of the Netherese ... and who else again?  I mean if it was just the Netherese, wouldn't it be the Forgotten Realm?  And they can't be all that forgotten, I mean people are always trying to dig their stuff up when there's a trace...

I kid.  Regardless, your take is poetic conjecture of a consumer I guess, maybe extemporeously your own waxing or maybe apocryphal lore.  There's nothing wrong with either, it gives games playing with that notion a bit of mystique to the game world they're engaging.  But if you want to have an answer as to what Forgotten Realms meant to the creator of the Forgotten Realms.  Ed Greenwood, who created the Forgotten Realms and called them such well before they were an official TSR product, is on the record as to saying way.  I'm sure you'd appreciate that there's a history of textual production, and it's probably best not confuse the materiality circumstances of that production, including branding, with presumptions that there was a deeper lore angle at inception than there was.   

But yeah, when the grey boxed set came out, people who were playing "in the Forgotten Realms" were very much thinking they were playing in Cormyr or Waterdeep or Sembia.  I mean all those gazetteers to the Forgotten Realms published ... few if any in the first run touching upon pre Dale Reckoning civilizations.

There is the setting of the Forgotten Realms and then there are the titular forgotten realms. Just like the setting Dragonlance and the weapon the dragonlance are two seperate things. It's not wrong to think of the present geopolitical entities as "the forgotten realms" because they make up the setting. But FR is, from the beginning, built upon these bones of lost high magic realms.

The question has already been answered, with multiple citations. The entire world of Toreil and everything in it has been forgotten by the people of Earth, here in the real world, and the only traces of it we keep are myths and folklore that have been warped and altered by centuries of retelling and reimagining. No specific realms have been explicitly forgotten by everybody else in Toreil, it's everything and everybody in the entire setting that has been Forgotten with a capital F for the purposes of the setting. You don't play a FR campaign set in an ancient kingdom that nobody remembers in setting because if that were the case there wouldn't be any lore on it due to nobody knowing about it. "forgotten Realms" means the Mystical Kingdom, Never Never Land, or whatever else one might call a fantastic alternate reality setting; Ed Greenwood called this one the Forgotten Realms because it sounds cool and whimsical for a game that is basically people acting out fairy tales in their heads.

Quote from IamSposta >>

If I recall, the Dungeons & Dragons animated series dug into the whole issue a little as those characters “fell through” a (and I”ll borrow from King for lack of a better term) “thinny” into the FR.

 No, the Dungeons & Dragons animated series predates the Forgotten Realms by several years.  The cartoon used a generic fantasy setting that wasn't based on any D&D campaign setting.