I think the point is that there's little reason to trust that you would not simply be robbed either by them leaving the garage open, or robbing it themselves.

If Amazon want to leave packages securely, then I am more then happy for them to partner with mail carriers and other delivery services and come up with a common standard for an externally secure lockbox system*. But they're not getting an open door into my house.

The problem in the delivery space is everyone does whatever - there's no standard or common code for communicating secure delivery logic for a premises. You can come up with whatever and it just won't be used. But "give me access to inside your private property" is one of the more insane solutions given that a garage is not an unvaluable area, nor necessarily a non-hazardous one.