The real reason (or the least cynical one) is that product reviews should not be used to review the seller/fulfiller/courier/etc. (because a single product may

The real reason (or the least cynical one) is that product reviews should not be used to review the seller/fulfiller/courier/etc. (because a single product may be offered through many combinations of those, and you're supposed to be reviewing the product itself). There's a separate place in Amazon's UI for rating/reviewing sellers, though it's not as easy or obvious to find (at least on mobile).