It is and it isn't a separate company. It's a GSE - Government Sponsored Enterprise. Think of it as a perpetual charter to engage in a specific business on behalf of the government.
The flip side is, UPS and FedEx can refuse to deliver to certain addresses. The USPS can't.
tldr; I hate junk mail and will scream into the void about it.
I guess they have to make money somehow.
USPS and FedEx are nice, but are more expensive than the USPS. And there's that whole rain, snow, sleet, or hail thing.