Those relics belong to dead people.

No, it belongs to a community. Does something stop belonging to a people if the original creators die? No.

That way nobody owns any land, because it belongs to the amoeba.

Returning the artifacts is meant to be a good will gesture, and a sort of a reparation (in lieu of the actual reparations) for all the horrible colonial era crimes that were propagated not more than even 100 years ago.