On a couple different levels:
1. Some people (like libertarians) have an ideological opposition to government-provided services, and believe all or nearly all services provided by the government should be abolished and replaced with the "free market."
2. An ideological belief that private business is "efficient" and government is "bloated and wasteful" by definition.
3. An lazy belief that "the government should be run like a business," therefore the tools you use to evaluate a business (like profitability), should be used to evaluate government.
4. etc.
I think those are distinct beliefs, but they kind of feed into each other and support each other. Like if you believe government services are wasteful and business efficient, it would follow you would improve things by abolishing the government services.
This is achieveable with “If you want to be licensed to deliver postal services, you have to deliver everywhere without discrimnating on price”. And the free market will find ways to make that work.
In any event, the US Constitution requires a post office.
US Constitution, Article 1, Section 8
> The Congress shall have Power ...
> To establish Post Offices and post Roads;
https://www.archives.gov/founding-docs/constitution-transcri...