PanAm had domestic flights too, just less of a network. Basically the CAB started carving up the nation’s air service in the late 20s/30s based on air mail routes, and at the time PanAm just focused on int’l service (with flying boats and everything) whereas TWA at the time was flying some domestic air mail. As air service grew around the country before and during the war, airline sort of consolidated into three categories

• flag carriers (like Pan Am, TWA and Northwest Orient)
• major carriers with nationwide (north south and east west trunk routes (United, American, Eastern,!Braniff)
• local service carriers that focused on a specific region (Ozark, Mohawk, Republic)

So basically PanAm didn’t care about or want a domestic network because the nationwide networks already existed and they chose to ignore them for the prestige/expertise/brand value of being THE name for US int’l travel. And during regulation it was totally fine because the CAB controlled and regulated all routes and all airlines had interline agreements and a passenger could somewhat seamlessly (not like today but the airlines cooperated a lot more because they were regulated to do so) go from local to major to flag carrier for intl travel. This obviously changed with deregulation and PanAm’s lack of a domestic route network hurt them badly (among other things). Their acquisition of National Airlines in the early 80s was supposed to help but by then PanAm was not exactly a finely tuned watch and was not cut out for the cut throat 1980s airline competition deregulation heralded.