I just switch ActiveItem, why view occur Loaded event every time?

[ahdung]

my MainView.xaml:

<Window>
  <ListBox ItemsSource="{Binding PageTypes}" SelectedItem="{Binding SelectedPage}" />
  <ContentControl s:View.Model="{Binding ActiveItem}" />
</Window>

MainViewModel.cs:

public class MainViewModel : Conductor<IScreen>.Collection.OneActive
{
    public ObservableCollection<Type> PageTypes { get; } =
    [
        // some Screen types:
        typeof(FooViewModel),
        typeof(BarViewModel),
        typeof(XXXViewModel),
    ];

    public Type SelectedPage
    {
        get => field;
        set
        {
            if (field==value)
                return;

            field = value;
            ActiveItem = GetOrCreatePage(value);
        }
    }

    IScreen GetOrCreatePage(Type type)
    {
        if (type is null)
            return null;

        var page = Items.FirstOrDefault(x => x.GetType() == type);
        if (page == null)
        {
            page = (IScreen)_container.Get(type);
            Items.Add(page);
        }

        return page;
    }
}

So I click item in ListBox to switch different views, why switch back to already created view before, it always occur Loaded event?

[ahdung]

I think i know, when switch ActiveItem, old view(a UserControl) removed from VisualTree, so it occur Unloaded, when re-show it, it need Loaded, it is WPF thing not related stylet.