Why doesn't Header binding DynamicResource work?

Question

Why doesn't Header binding DynamicResource work?

<DataGridTextColumn Binding="{Binding CustomerID}"    Header="{DynamicResource firstName}" />

code environment：

App.xmal:

<Application.Resources>
        <System:String x:Key="firstName"  >Before Change Name</System:String>
        <System:String x:Key="secondName" >After Change Name</System:String>
    </Application.Resources>

MainWindow.xaml:

 <Window.DataContext>
        <local:ViewModel/>
    </Window.DataContext>
    <Window.Resources>
        <DataTemplate x:Key="headerTemplate">
            <TextBlock Height="50"  Text="{DynamicResource firstName}"    TextWrapping="Wrap" />
        </DataTemplate>
    </Window.Resources>   
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition Width="200" />
        </Grid.ColumnDefinitions>
        <DataGrid x:Name="dataGrid"   ItemsSource="{Binding Orders}"   AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTextColumn Binding="{Binding OrderID}"   HeaderTemplate="{StaticResource headerTemplate}" />
                <DataGridTextColumn Binding="{Binding CustomerID}"    Header="{DynamicResource firstName}" />
            </DataGrid.Columns>
        </DataGrid>
        <StackPanel Grid.Column="1">
            <Button x:Name="btn" Content="Change Resource"     Width="150" Height="30" Click="OnChangeResourceClicked"/>
        </StackPanel>
    </Grid>

Click Method:

 private void OnChangeResourceClicked(object sender, RoutedEventArgs e)
        {
            App.Current.Resources["firstName"] = App.Current.Resources["secondName"];
        }

Result after clicking the button：