There are various reasons. What if it turned out someone was using a stolen ID or a fake ID, or the ID didn't match the face, or it wasn't even an ID? You'd want to be able to see how your process missed it.

The real problem is that there aren't many options for real authentication over getting people to upload pictures of high-value credentials. Now every service has to be a security expert, like encrypting the images at rest so they aren't the ones who leak it.

It's kind of like how dumb our credit card system is where you have to both share a secret with everyone (from random websites to random restaurants) while hoping the bad guys never get it because the secret can be used anywhere. It kinda works against everyone except the bad guys.

Maybe it's time we come up with a deliberate system.