Why cannot I stretch items horizontally when using GridSplitter?

I have a Grid with two GridSplitters:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="1*" />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" MinWidth="200" />
        <ColumnDefinition Width="5" />
        <ColumnDefinition Width="Auto" MinWidth="200" />
        <ColumnDefinition Width="5" />
        <ColumnDefinition Width="1*" />
    </Grid.ColumnDefinitions>

    <!-- Financials -->

    <Border Grid.Column="0" Grid.Row="0" HorizontalAlignment="Stretch">
        <ScrollViewer HorizontalScrollBarVisibility="Disabled" VerticalScrollBarVisibility="Auto"
                  HorizontalAlignment="Stretch">
        <ItemsControl ItemsSource="{Binding Financials}" HorizontalAlignment="Stretch">
            <!-- ... -->
        </ItemsControl>
    </ScrollViewer>
    </Border>

    <!-- Splitter -->
    
    <GridSplitter Grid.Column="1" VerticalAlignment="Stretch" Width="5" />

    <!-- Actions -->

    <Border Grid.Column="2" Grid.Row="0" Background="Red" 
            HorizontalAlignment="Stretch"/>

    <!--
    <ScrollViewer Grid.Column="2" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <ItemsControl />
    </ScrollViewer>
    -->

    <!-- Splitter -->
    
    <GridSplitter Grid.Column="3" VerticalAlignment="Stretch" Width="5" />

    <!-- Simulation -->
    
    <ScrollViewer Grid.Column="4" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <ItemsControl />
    </ScrollViewer>

</Grid>

However, regardless of how do I configure items inside the grid, I cannot make them stretch horizontally:

What am I doing wrong? The live property inspector shows, that both root Borders have HorizontalAlignment set to Stretch, yet neither is actually stretched in its column.

Add a HorizontalAlignment other than the default Stretch to the GridSplitter declarations:

<GridSplitter Grid.Column="1" Width="5" HorizontalAlignment="Center"/>
...
<GridSplitter Grid.Column="3" Width="5" HorizontalAlignment="Center"/>