Why must I install High Sierra on my Mac Pro 2012 running Sierra before being able to install Mojave. My ideo card is Metal Capable?

My guess is so you can check your card for Metal capability.

1. Press and hold the Option key while choosing Apple () menu > System Information.
2. In the sidebar, select Graphics/Displays. If your graphics card is
3. compatible, you see "Supported" next to the Metal entry. You can also
4. note the name of your graphics card and see if it's in the list below.