What are you talking about?
Why would you get an electronic book through physical mail? That’s completely retarded. Does the USPS really do that?
Blind people have a hard time using the net.
There are millions of others who can't access the net (blind for example). They order lots of things by mail.
Then there are electronic books produced in languages rarely found in use in the US, and probably not accessible on the net even if you wanted to do it that way, and those folks who want those books order through the mail, and receive appropriate recordings through the mail.
Remember, your experience is not necessarily the universal experience ~ however retarded you believe that universal experience to be. It's a huge planet ~ even though literally everybody eats frozen chicken produced in the US, if they're lucky.