> ...in fact, it's so much a better solution that I can't imagine Amazon hadn't considered it.
I agree that it seem a better solution to the one scenario you're mentioning: delivering packages.
But if you watch the video in the article [1], they describe other scenarios: "maybe let in your dog walker, a friend, or maybe to let in a team of cleaning ninjas"
I don't think it's too far-fetched to see the 'friend' here being actually your airbnb guest. Many airbnb homes already use a rudimentary lockbox with code, if the guest can now instead set up something on their amazon account and know they'll be able to get in (at least the initial check-in, and have a key waiting on the coffee table), it could make the experience smoother for the guests, and the host as well.