Because the US Postal Service is viewed as a utility, and required to service every address in the continental US (as well as most of its territories). Fedex, UPS, and other private carriers are under no such obligation, and so can extract profits from the most profitable routes or services, leaving USPS with more less profitable services to provide (first class, bulk mail).
If you don't think the USPS provides a phenomenal service for the price they charge, you're insane. I can send an ounce of paper across a continent for $0.50-$1.09.
The last mile of shipping is the most expensive. In the US, there are tons of last miles that are thousands of miles from very large cities.
Switzerland is a tiny country relative to the US, so it's much cheaper to deliver mail.
How much cost/time does it cost Swiss Post to ship a package 4000km ?
It's at least 3x the price of USPS:
https://www.post.ch/en/private/a-z-of-subjects/prices-sendin...
I read your comment as 'Given USPS can send a package over 4000km in the same country, let's compare that with international charges in Switzerland'¹. Is that correct? Because that would be a liiiittle bit misleading in my world and I'd like to compare Swiss international charges with a USPS delivery from New York to Reykjavik please.
① Apologies if that isn't the case. But 4000km just felt too close to the east coast to west coast distance.
There's also a number of alternate services for the post in a number of other countries. FedEx is not a duplicate of the post - it's for sending small but high value things extremely fast. And between FedEx One Rate and FedEx Smart Post, it's pretty clear that they view USPS as a competitor to be respected.
http://www.wsj.com/articles/u-s-mail-does-the-trick-for-fede...
Because as far as I'm aware, Switzerland opened up to DHL et al. I'm not sure about mail/letters tbh.