Do you not have a housekeeper, or a landscaper, or dog walker, or anyone else who goes into your house when you're not there? It's pretty common for service personnel to be in a house doing stuff.
[I'm unsure why a landscaper would need to enter your home or apartment or whatever.]
I grew up about an hour from the nearest UPS facility. A closer one still doesn't exist to this day. I didn't have to worry about people stealing a package from my house, but if you go five minutes into town, you get to neighborhoods that are much more dense. If you wanted UPS to hold your package, you had a 2 hour round trip to get something you paid to have delivered to your house.
I'm not a strict advocate for Amazon Key, but I can definitely see the use case for it.
You'd think you could just fill in a textbox with a description of where the lockbox is when you make your order, and Amazon could forward that information to the delivery company.
My case is atypical though because I installed it on an outer gate, meaning delivery people are not actually entering my home.
And that the feature also lets those that want to just to give access to their whole house seem a useful way to grow the adoption (since my guess is many more people will take this more convenient, though less secure option).
That's not necessarily as easy as you seem to be making out, a lot of people don't have cars which rules out large or heavy packages and some delivery offices have inconvenient opening times (often business hours + Saturday but if you already have recurring commitments on Saturdays you might have to go through a lot of effort to make the time during the week).