It's zero cost and zero effort to port someone's number out, or get a new SIM card issued for the existing account.

I've worked with a bunch of streamers and YouTubers, and the threat model is such that people have shown up with professionally made printed fake IDs to attempt hijacking in an actual retail carrier store.

It's an attack on humans, not on technology. You trick their phone carrier's employees into thinking that you're them and that you lost your phone. Then you end up with a SIM card assigned to their phone number, so you receive all of their calls and texts instead of them.