Actually, they've recently done that, but just have not publicized it.
Use an HDMI, DVI, or VGS adapter is the section to look at. And I asked them a week ago to fix the link to the Microsoft branded cables but they didn't. The link is https://www.microsoft.com/en-us/store/d/surface-mini-displayport-to-hdmi-20-adapter/8qj8ss92m2tl/ch5z?activetab=pivot:techspecstab
>I sure wish Microsoft would just state that their Dock does not support 4k monitors and you need to use an Active adapter.