Wpf Button Template: Border without background not working [duplicate]

Question

I created a button with a corner radius based on this stackoverflow answer, which works fine.

Here is the XAML:

<Window x:Class="BorderButton.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:BorderButton"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <Style TargetType="Button">
            <Setter Property="Margin" Value="5"/>
            <Setter Property="Foreground" Value="Black"/>
            <Setter Property="Height" Value="150"/>
            <Setter Property="Width" Value="150"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="Button">
                        <Border CornerRadius="5" 
                                BorderBrush="Black" 
                                Background="{TemplateBinding Background}">
                            <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center" Margin="5"></ContentPresenter>
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center" >
        <Button Click="Button_Click">Button 1</Button>
    </StackPanel>
</Window>

Question: If I remove Background="{TemplateBinding Background}" the button click event only works if I exactly click on the button text. If I perfom a click inside the border area (and not at the text) no click event is fired. What is the technical reason for this behaviour? Why does border need the Background property set to work correctly?

Thx