Because afaik, USPS scanning is not the same as UPS/FedEx scanning. It doesn't seem as sophisticated or precise, at least from my own experience tracking USPS. It used to be that they would only scan an item when you dropped it off and when it arrived at your local post office, and any time between that just said "In transit." Even though they've improved their tracking system, there will still be long periods (durin train or truck transit, for instance) where the tracking updates go black for a couple days.
You have to remember that USPS processes more mail in a week than UPS handles in a year. Pinpointing one single package is going to be a lot harder for USPS than UPS/FedEx. It goes back to that old saying, you get what you pay for. UPS and FedEx may be more expensive but they are usually able to track your package down to the truck/sorting facility it's in. Of course, this doesn't mean there aren't issues with those companies, but at least if a box is lost or delayed they can tell you where it is most of the time.
USPS does receive a lot of flak and sometimes you can't really put blame on them. As you said, their traffic is immense compared to the others. However, if you're going to pay close to if not the same anyway, I would bet with the company who has shown better success. A lot of shippers will only ship USPS and others will avoid it completely. As Calihawk mentioned, proper packaging is a must. I find hold for pick up when using USPS to have much better success as well... But that's assuming the package makes it.
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