The USPS does thing no private service – including the ones DeJoy is invested in (yes, of course he’s openly in a conflict of interest! Who appointed him?) – will do.
It’s parallel to public schools: they serve everybody, not just those who can pay for it. Or perhaps it should be compared to the army: it is a service, not a for-profit business.
The other systems get to exclude addresses. The USPS does not. You can send several ounces of letter to ANY address in America for the same low price – far, far, FAR lower than you would with FedEx or UPS.
And they would be fine financially had not Congress imposed some vicious requirements not found in any other government service.