In the past, post offices were more social hubs than they are now, since it was the one place in town where you could receive communications from others, unlike today with email, text, and DMs being available.
Being from a rural town, everyone knows who works at the post office, and many times you do stay awhile to chat with the workers there, and even those that have come in. I live in a larger city now, almost always use the self-serve kiosk or have postage on my mail so I just have to drop it off, so I don’t spend much time in any of my post offices these days.
You may run into a friend when mailing a letter or package, or even go there for services other than mailing. For example, many US post offices offer passport photo services. They may even sell greeting cards and packaging supplies in some, if needed.
I think it’s more so considered a social hub with Niantic in the historical sense, but it may also be one of the few social hubs in a rural area. I know this is what some of the ambos have said on the subject in the past.
Also, some post offices in larger areas may be located at places that may be social hubs themselves, like a shopping center or a grocery store. The northend post office in my city is in a shopping center, which is also home to a grocery store and other stores.
As for UPS and FedEx locations maybe being eligible, this would be tough, as they are private chain businesses, whereas most post offices are government run and always accessible to the public. I doubt UPS/FedEx, even Amazon, would want the public hanging around their distribution centers, which may only have a small space open to the public, or only open to employees.