If you experience this problem please do the following:
1. Turn off anti-virus completely. (This cannot be stressed enough as AV programs have a tendency to block games from running without any kind of message)
2. Verify integrity of CK2 game within Steam app.
3. Set override_resolution_safety = yes in settings.txt in the CK2 folder under My Documents/Paradox.
4. Disable fullscreen optimizations.
5. Start game.
MAC SPECIFIC SOLUTION
CK2 stores graphics files in Documents/Paradox which means macOS uploads those to iCloud & removes them from the hard drive if it needs space. Go to that folder in Finder, re-download them by clicking the cloud button. (courtesy of @epaga).