Because it is monopolized by the government.
Discussion
I'm sure there are other reasons. The monopolized US military has a reputation of being effective.
Perhaps we should consider how much budget the U.S. government allocates to the military each year compared to the postal system. Additionally, demanding both low prices and high quality is somewhat unrealistic.
Very true. Then the PO ends up with a bad reputation, but how many others would think this deeply when aligning your product with the PO?