Ignorance or denial. Most eventually do register their music.
It’s a mess, and it’s happened many times over. When a track is fraudulently registred, buyers get copyright claims they cannot clear. When it happens, the author cannot help them. They have to fight the fraud first. Though it’s a pain for everybody, I haven’t heard stories about an author not being able to eventually reclaim their own music.
Clearing the claim through the third-party clear form usually takes a few hours, not a week. The license people buy is precisely the key to clear such claims… which is why they buy the license.
Edit: This is about ContentID, but I haven’t seen your question about PRO though. You should check out this guide about ContentID.