Having sources in Amazon, I can tell you the goal here is not simply to deliver Groceries but rather to find a way to remove FedEx / UPS / etc from their cost of goods sold.

The economics of Fresh are very poor for Amazon and likely will always remain so, unless they can start to leverage it for same day shipping of the rest of their product line.

It's not something Amazon likes to discuss too much as it has the potential to upset their partners significantly in this space.