You say that like it's a bad thing.
Why does a hill billy who insists on living at the bottom of the Grand Canyon deserve a subsidy? That public money would be better spend helping poor people.
I'm going to say this nicely once instead of the reply I really wanted to say because I'll give you the benefit of the doubt of not knowing or looking it up.
Because it provides postal service to the Havasupai people living on their native land at the bottom of the Grand Canyon. Would you like to take a guess as to why Supai, Arizona, the capital of the Havasupai Nation, is located at the bottom of the Grand Canyon.
EU countries privatized their postal services decades ago, because governments are not allowed to compete with private entities in the market (unless explicitly allowed by EU-wide laws). And because the idea felt good, the same privatization extended to territories outside the EU, such as Greenland.
There is daily USPS service to a postbox at the bottom of the Grand Canyon that is only accessible by mule paths. I guarantee this service would either be cancelled or go up in cost to thousands of dollars per letter if USPS was privatized. The sheer size and remoteness of parts of the USA is why it's a public good.
https://facts.usps.com/8-mile-mule-train-delivery/