Seems the aspect you are confused about is slavery, aka the overarching plot of the game.
Elves are kept on Syl'anar because if they are introduced elsewhere they can escape, breed, and rebuild their society as free people. This has been mentioned many times in game.
As also highlighted in game, Trenero's project which had him export a bunch of elves to The Capitol (on another continent), elves escaped and intended to do exactly as mentioned.
The humans had them neutered though, horrifying as that is.
There is no one to offer literally millions of elves asylum, because the only other people out there are humans, who enslaved them in the first place.
There is one other type of people out there though, which if you've played the game you know exactly who and why they stay away.
The MC - aka you - grew up near The Capitol, which as mentioned yet again, has no elves. Hence slavery being such a foreign concept to them (you) as you go through the game. That's brought up even more often.