Several reasons. It would work with any store or even fast-food/restaurants, sending my car to pick up a burger would be fantastic. Human employees walking the packages to your car (which would have a special lane in front of the store) could be easily replaced by a robot, at the moment it's just cheaper to have a human do the task. For many people who need a low paying and low skill job, that would be a great opportunity for them, there wouldn't be any dealing with customers either.
How would the autonomous delivery trucks you are talking about actually deliver the packages? The very difficult part would be getting the packages from the vehicle to the person's door. It would not work if it required the customers to come outside to pick up the packages, people wouldn't be home, they might be busy at the moment, it would be difficult to ensure they don't take the wrong or someone elses packages. I've considered that before and I just can't figure out a way for that to work.
The drones are also impractical and could only be used for small items. You would basically need a landing zone in your yard, I just don't see that happening nor being part of the free shipping. In my opinion the drone announcement was just a research project (that was full of flaws that can't be overcome) or a marketing stunt.