Why isn't mobile billing listed as an available payment method in the United States when my carrier offers this to it's customers and my spouse already uses it with his Samsung device?

Everyone here is a user. Apple is not here. Apple doesn't answer questions here. No one here can tell you why Apple has made a certain business decision.

ASC - Late 2012 Mac mini, macOS 10.15.2; Watch, series 5 GPS/cellular, watchOS 6.1.1; iPad Air 2, ipadOS 13.3; iPhone 8+, iOS 13.3; TV 4th Gen, tvOS 12.4.1;  HomePods, audioOS 13.3 and Airport Express w/ Airplay 2