It's in the Constitution as a service the government is to provide.
It is authorized or provided for - things like “post roads”, but that doesn’t make it required does it?
The postal service and armed forces should be the only thing the federal government should be in charge of.
The federal goverment has become a place for people to get their personal reparations for whatever way they feel the United States has slighted them. It is why we have a deep state.