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."
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.
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...
However, what I believe many are missing is that having a physical mail network that is accountable to the public good and our government is an advantage for many important situations that should not be dependent on the free market.