Why does SharePoint 2016 Copy option only show Base URL of the Library?

In SP2019 modern view, after you click Copy To or Move To, the right panel will prompt up and user can browse into subfolder (and to create subfolder here).

But you cannot select other libraries or site in the panel. The operation is limited to current library only.

In SharePoint online, the UI are the same but you can select other sites and libraries in the same tenant.