Well if you look at the map Faerun only seems to be on of Torils many continents....which implies most of the world at large is unknown or cut off and hence Forgotten.
Plus many of the remote places on toril Kara Tur, Hordelands, Unther, Maztica, and so on and so forth are all based on ancient cultures... so again Forgotten or ancient in nature.
Much of the lore and and history of Forgotten Realms is lost and hence forgotten...with Netheril and greater heights of magic having once existed, but few if any remember or know about it....yet traces of it are all over Faerun....so again forgotten.
As for a portal to earth... I do seem to recall some Dragon sections that had Elminster, Dalamr and one of the Seven from Greyhawk (Mordenkainen?) coming together to share ideas, spells, knowledge of magic items....etc... and Elminster once served them all German beer and Microwave lasagne, which he almost certainly got from earth.