Because we used to say, on the air, "I'm good in the Callbook". The ARRL used to publish a call book of all FCC licensed AROs with address, which was used for QSLing. QRZ provides this information, and a whole lot more, for free, at the tap of your fingers. Remember in the movie The Jerk, the Steve Martin character ran through the neighborhood shouting, "The phone books are here, the phone books are here", because he had his first telephone and his phone number would be listed in this new edition of the phonebook. Ditto that for new hams and a new edition of the Callbook after they were first licensed or upgraded. … At least everyone checked to make sure they were, "Good in the Callbook". Even an old, outdated Callbook from a previous year was a treasure to have and use even though it was not current. It was likely most of your contacts would be in your old Callbook. If you needed QSL info on someone not in your old Callbook you could go to the library and use their Callbook.