Here's the thing (other than basic enshittification):
For a lot of rural and wild (mountain and desert) places in the US, USPS covers the last mile (or last 100 miles) that no other service does. At a loss.
Private delivery service gives no fucks. We know this because UPS and FedEx or any other hand their parcels to the USPS pony headed out beyond the pale to keep their oath.
If we privatize, then those places vanish. They also stop paying taxes.