How come Elsa and Anna don’t know about their kingdom’s traditions?

There are natal trees and decorations in the castelo even before Olaf sets out to find traditions. The sisters had to have known a thing or two about natal traditions. And why does Olaf try to take all the things he "borrowed" back home. Shouldn't the sisters already have most of these things available to them? They are royalty. Maybe it would be better to give these things to those mais in need of them