There isn’t a legal restriction
Which is to say they could show replica’s if they wanted to; they just don’t want to. This is fairly typical - art museums display art, not reproductions of art. They also tend to be interested in displaying the art they have, not telling the life story of the artist.
If a country were to hypothetically set the length of time for a copyright to expire (and thus enter the public domain) to one second, does that make it legal to freely distribute any work of art produced in any other country at any time, within the borders of that country?
If in your example you assume the country is not subject to any treaty obligations ...