Maybe it's the seller who requests that the home not have a lockbox.
I had a seller concerned that someone would swipe the key. He actually said that he'd prefer just to be called and then leave the home unlocked during the appointment times. They live in a low-crime area and if I was living there I wouldn't be worried about someone invading the home during a short time window. I guess might be more concerned about having the key swiped and having to re-key the entire house to feel safe once it was gone. But then again, I'd be MORE concerned about not making it convenient for buyers to preview the property.