Most interaction with USPS is at (residential) mailboxes. Sure, post office service is not great but they do a decent job of moving letters, post cards, and packages in a relatively timely manner - and the service for letters and postcards is damn cheap. In now remote / rural areas of the US, no for profit company would bother providing service, as it is wouldn't be profitably - but USPS serves everyone.
The fact that the USPS is currently operating at a loss has a lot to do with a bill that passed in 2006 requiring them to prepay their employee benefits including pensions 50 years in advance.
