Why does iCloud Drive get stuck?
All programs are supposed to "just work", however bugs are unavoidable when humans are programming. 

With a brand new install it seems that logging out or rebooting the Mac to test if this fixes the problem shouldn't be a problem. So you might want to give that a try. 

If you want to fix the problem without logging out or rebooting, try opening a terminal and running:

brctl log --wait --shorten


This should give you a reasonably readable output of log info from the syncing process. Hopefully this will give you a clue as to why the process isn't progressing.

You can also try simply killing the bird process that handles the synchronization. When it starts again it might progress to completion.