They're private and have to satisfy customers and the USPS doesn't give a rip. They stay in business no matter what. Just look at the massive losses racked up by the USPS.
I have been a Realtor in the panhandle of Florida for over forty years. I go back to when the only way I could get documents to, say Miami and back in two days was, 1. Drive them there and back (1 day, 1,000 miles) or 2. Put the documents in a manila envelope or equal. Take to the local Greyhound bus station. Pay an enormous fee. The Miami bus then takes the envelope to the designated Miami post office. The recipient then went to the bus station, picked up the package and took it somewhere to find a notary, sign, witness and notarize the documents. Back into an envelope, back to the bus station, pay another fee, a bus takes envelope back to the Panhandle bus station. The agent goes to bus station picks up the envelope and takes it to the closing. Instant closing, only three days MAYBE two.
The USPS is much like all other brick and mortar businesses which were either driven out of business or were dragged kicking and screaming to providing additional services.
So far behind was/is the USPS one day service is that they lease space on FedEx and UPS aircraft.