The Post Office wants to make sure these mail repositories meet strict requirements since they become the property of the Post Office. So they subject these to extensive testing before they will approve the manufacturer to make them.
The Post Office also requires curbside mailboxes to be approved by the Post Office. These are a little different than the central pods in that they do not become the property of the U.S. Post Office. For example, the MailCase locking mailbox that you purchase from MailCase would not become the property of the U.S. Post Office, it would remain your property. If you moved to another house, you would have the choice of leaving the locking mailbox, or taking it with you to your new home.
However, these curbside locking designs still have to meet extensive specialized requirements by the U.S. Postmaster General.
Many homes in older neighborhoods still have mailboxes attached to the house. These mailboxes are actually not required to be approved by the U.S. Postmaster General. However, as locking mailbox designs become more prevalent, I think the Postmaster General will publish a standard that these mailboxes have to meet.