It is the law. Postal Service Reform Act of 2022 which mandates that the United States Postal Service (USPS) deliver mail six days a week.
Postal Service can't change it on their own, it has be changed by the Board of Governors and Congress.
They have looked at changing it before, but can never get it past the powers to be. In addition, most other countries, the postal service is ran as a business and not a government service. They have some that are actually large banks as part of the service they offer. Again, by law, the Postal Service is limited to what it can and can't do.