And Amazon couldn't set themselves up for delivering at the same times?
They could, but their business model is set up to make margin from (promised) faster delivery, and they are completely blind to the idea that some (many?) customers don't mind having to wait a day or five, but would really like to be able to specify the time of delivery rather than the date.
Obviously some customers would really like control over both, but Amazon haven't twigged that either (nor have most delivery companies).