Why doesn’t WPF border control have a mousedoubleclick event?

Just use InputBindings.

<Border>
    <Border.InputBindings>
        <MouseBinding MouseAction="LeftDoubleClick" Command="..."/>
    </Border.InputBindings>
</Border>

In general; avoid using events if not developing controls in WPF. Usually the usage of code behind based events is a strong indication for a MVVM Pattern break.