Living in Norway where the mail system is undercharging massively in comparison with other countries, yet delivering flawless deliveries 99.999% of the time, the fact that Americans still use USPS is baffling to me.
I'd love to see where the stats on Posten Norge show 5 9's for reliability. Also, while I can't find updated stats -- according to the BBC back in 2012
it was the most expensive
of the 65 countries they surveyed.
That said, the answer to why most people in the US still use USPS -- it's really, really cheap and relatively reliable. A first class stamp is just under 50 cents. Most bubble mailers can be shipped anywhere in the country for under $5. Sure, the parcel fees are more expensive (smallest boxes are $5 and fixed rate tops off at $20) but they're still dirt cheap. Compare that to 140NOK for a small parcel in Norway -- I believe with the exchange rate that's around $15 now?
In my experience, the majority of my issues with the USPS have come from the first leg or last leg. This really depends on the quality and efficiency of the local post office. Given that the US is 9.8 million km2 and has 320 million people, the scale they deal with is a few orders of magnitude larger that of Norway's postal system and also therefore subject to larger variability across the country.
Combine the facts that the postal workers have to visit every household 6 days/wk and that the average household is getting more parcels (thanks Amazon but really thanks ePacket -- another huge source of loss) but less daily mail (since the peak in 2006) while Congress has been forcing them to increase spending but still balance their budget -- it's no surprise that USPS employees are overworked and stretched way too thin (ever heard the term "going postal"?). The bandaid solution of hiring "transitional" mailmen year round to deal with dense urban populations relieves stress from the USPS but also decreases reliability (both of the stolen packages I've had this past year were stolen by one of these temps).
Politics aside, USPS is the classic example of you get what you pay for. If us Americans were willing to spend more on a government service (which we generally hate doing) then it'd probably increase in quality but instead we want it both cheap and as reliable as it was when the system had to deal with half the volume.