The reason is that the software that FedEx, UPS, etc use to plan routes doesn't have something which maps P.O. box numbers to the street address of the post office that they are contained in. And from what I can tell the USPS doesn't publish that database, so even if FedEx, etc really really wanted to, they couldn't without spending several million (or more) to map them all manually.
It's unfortunate that it's not terribly convenient, but once you know the street address of the post office that your P.O box is located at, it's not a problem at all.
The reason is that the software that FedEx, UPS, etc use to plan routes doesn't have something which maps P.O. box numbers to the street address of the post office that they are contained in. And from what I can tell the USPS doesn't publish that database, so even if FedEx, etc really really wanted to, they couldn't without spending several million (or more) to map them all manually.
It's unfortunate that it's not terribly convenient, but once you know the street address of the post office that your P.O box is located at, it's not a problem at all.