That used to be the case, and was one of the things the USPS used to complain about--- they were legally mandated to deliver to every valid postal address in the U.S., while UPS and FedEx could exclude unprofitable remote areas. It appears both UPS and FedEx now promise delivery to every U.S. address; would be curious to know when that happened. They do, unlike the USPS, levy a surcharge for delivery to areas deemed "remote", though.