The races showing in the slider aren't the actual races that will appear in the game. Don't ask me why it's handled that way, but in normal Stellaris play they don't really have static maps and prescripted empires handled the way we do. That said, I'm hopeful that a solution may be coming in a future Stellaris patch.

If you start the game and switch to observe you'll see what I mean.

Awesome, thank you!