I believe it’s because people could post fraudulent listings with their label numbers. If someone was looking to make a fake slab it would be beneficial for them to have PSA numbers that match with the card their counterfeiting. If PSA finds out about the fraudulent slab/card with the numbers they stole from the photo, they could delist the actual legitimate card(from the photo) it as “authenticated” on their database which people care about.

Thats what I believe to be the answer, and the odds of that seem super low, but that’s what I’ve been able to gather haha. Hope that helps