[dragonwriter]

> Im not American, so might be missing some context, but what is the monopoly?

There’s a monopoly on residential letters, and a monopoly on mailbox access:

https://en.wikipedia.org/wiki/United_States_Postal_Service#U...

> You can use Fedex, UPS etc to ship anything from anywhere to anywhere.

You actually can’t, there are very specific rules for what you can use a non-postal carrier to send to which addresses and at which prices. Which is why you don’t see general, non-express, or cheap letter delivery from third-party services.