You can do that, and I'm sure people already do. But you can't truly test things at scale. Are OCSP servers going to get overloaded? What about IoT devices and things out of support? You have deployments which configure their systems in non-standard ways, maybe some corporate manager decided to mandate cert pinning.

The original comment was to do a fire drill and revoke live certificates to force everybody to participate in the test.

You can have a fire drill and evacuate an office building, but you can't hold a full-scale forest fire/volcano/hurricane drill and evacuate 3.5 million people to test mass evacuation routes, and also you got the edge cases that don't participate in drills, like homebound seniors or hospitals, where disaster strikes.