I’ve never played or run a Forgotten Realms campaign, but it’s more or less synonymous with D&D setting now, from the movie and BG3. (I run Greyhawk and DM’s I’ve played with generally do their own setting, if any.)They have really been moving away from the Forgotten Realms as the base assumption for a few years, amd having a general description and map in the DMG means they can say "the default setting for this Adventure is The Pale, but in FR you can set it here, or In Eberron there" would be a pretty sensible approach.
Any idea why WotC wants to change that?