A theory about the mysterious fake sky in the Kirby and the World Beyond trailer...

So, here's something bugging me... In many of the Kirby games, we explore parts of Popstar. But, only parts. Not the entire map... And since the Ancients lived in Planet Popstar before, why haven't we seen the many ruins that they left? What if the area where Kirby lives is just a dome to hide the outside areas...?