Actually this is a bit of a myth.
No, it is not a myth. Not. At. All.
The Realms really is a kitchen sink setting. It's certainly gone through phases where elements from other worlds have been deemphasized, but the concept of the Realms being connected to other worlds predates its publication as an official D&D setting.
Not ready to believe me? Ask Ed.
This has been reflected in several D&D novels--and not a little bit of official material in Dragon and Dungeon magazine, too. Jander Sunstar ended up in Ravenloft. Mages from other worlds have long crossed paths with mages of the Realms. Myth Drannor was once a planar crossroads. One can travel from certain portals in Undermountain to any of several different D&D worlds. It's all there. It's all official.
Its this core concept of the Realms that's led it to become a storehouse for all kinds of elements from other game worlds, as well as the backdrop on which DMs and players across several editions of the game have been expected to place all the shiny new toys (read new game rules for players, NPCs, magic items, dungeons, adventures and monsters) found in each newly released sourcebook.
This doesn't mean you should expect to see Knights of Solamnia or Warforged on every street corner in the Realms. What it does mean is that a player (with their DM's approval) can play a warforged PC in the Realms without the DM having to go through significant contortions to justify the warforged PC's presence. It means DMs can borrow cool plotlines from whole other game worlds and port them into the Realms.
And it means that game designers and novel authors can, if the story calls for it, mix and match elements from other game worlds to place in the Realms.
I get that you don't agree with how it's being done currently, it's just that this is not something new.