Since AT&T claims to cover 97% of USA, what are the obstacles preventing 100%?

AT&T is only used as an example because of it's current commercials. There are other firms including potential new ones of course. What would be the necessary things needed to provide absolutely 100% telephone, internet, TV, etc., coverage? Typical short delays or degradation due to spontaneous atmospheric conditions should be ignored. Even the most sophisticated electronic equipment have short term conditions that need to be ignored without investing in expensive relay systems that would make fixing the problem with backup systems counterproductive because of the cost to the users.