I don't know what USPS service you have, but... if you have USPS service so exceptional that you find it to be truly better than all other alternatives, well, great, good for you. It just doesn't seem to mirror the experience that I and everyone else I know has.
Society determined (relatively, for the US) long ago that every resident must be able to have access to mail delivery services for a variety of functions, such as:
Civic responsibilities (e.g. voting, being informed of jury duty, &c.)
Delivery of goods (e.g. medicine)
Communications (e.g. letters for correspondence)
This not only includes city dwellers which constitute the vast majority of /., but also people hundreds of miles away from a major city (e.g. the entire states of Alaska & North Dakota, Navajo nation, &c.) There are areas between the US's borders with Canada & Mexico which have no other lifeline to the rest of the world and private couriers have no obligation to provide services to, so they outsource to the USPS.