Not everything on the planet is required to make a profit. The USPS is a service that reaches every single town, every single address in the country. We pay for the USPS for the same reason we pay for public roads: Because it's infrastructure. And quaint as it might sound, it's part of the social fabric that unites us as a people.
Those of you pointing to FedEx and UPS as models would do well to remember that those businesses are in no way interested in doing what the USPS does.
And as others have said, the USPS was recently set up to fail: The same political faction who crows, "See, the government just can't do anything right!" have set out to prove it, in this case, by hamstringing the Post Office. There's a lesson to be learned from that, but it has nothing to do with the Post Office.