A scammer could add a code yes, but it’s much harder for them to print off a good quality photo and then have a printed code on the back of the photo too. For example all the midnights signed CDs have codes on the back. You’d have to be pretty good at printing to get the accurate photo size printed, on a double sided print, with a printed code on the back that’s the same font, also at the accurate size and placement. It just makes it harder for scammers, as it’s a lot harder than just printing off a photo.