Running macOS from a physical partition

It would be nice to be able to run macOS (10.13) as a guest OS when already installed on another partition, akin to running Windows on a Bootcamp partition. I really don't want to have to reboot every time I need to access that partition (I am moving to a clean install but need info from my old boot volume). To me, this seems like running a VM from a Boot Camp partition, but Parallels won't do what I need.

I really don't understand why this works for Windows, but not macOS. Is this a problem with macOS or are the developers just unable to figure it out. Incredibly aggravating!

Tim King