Looking only at a package you receive does not even come close to the complexity of managing a supply chain of packages.

Shippers the size of Amazon are dealing with a supply chain of packages that is larger than the majority of businesses. They have likely already considered that they could ship in a different package, but it costs them less by having fewer packages to manage.

If every possible size/weight combination requires a different package, then the amount of packages sitting in warehouse will increase by an enormous margin.

I would wager that the actual resources consumed by having fewer package sizes to manage is less than the amount of resources consumed to have a huge variety of package sizes to manage.