And this is the real problem. USPS doesn't mess up very often. In fact, they have a truly outstanding record.
The real issue is that when they do screw up, they have absolutely no way of fixing it. Their attitude is like "Oh well, we can still track it, so I guess it'll get there eventually."

Example: my infamous 32 day package, it was only supposed to go about 400 miles from one end of my state to the other. Instead it "Looped" through 7 cities in 4 states multiple times. Every time I tried to get help, the answer was always the same: "We have the tracking. We know exactly where it is."
Ok, but what are you gonna do about it?
Answer: "Nothing, we're tracking it."
BTW: My package finally just appeared mysteriously in my mail box. I checked that tracking number for a long time after. For about 3 months, it showed as being "Enroute to next destination."
In a nutshell:
USPS normally does a good job.
When they do screw up, they won't admit it.
They have no way of fixing a mistake, so they just don't try.