That's kind of my point. I'm sure a lot of newer players have actually played in the Forgotten Realms without even knowing it. They don't know about the world, they just think they're playing through Tyranny of Dragons or Princes of the Apocalypse or Tomb of Annihilation. A lot of people probably don't realize that they're playing in that setting because it's assumed at the core of 5e.
And I bet the same thing would have happened if, say, Greyhawk or Eberron were the main setting of 5e. So it's not that the Forgotten Realms is actually more-liked than any other setting or inherently better at fulfilling the role WotC gave it this edition, it's just the one they chose and there are other settings that would have worked at least as well as the FR.