You would think, since the gov doesn't need to make a profit, that they would run FedEx and UPS out of business ... even without that "burden-of-making-profit" they are losers. Shut it down and put it on the museum wall right beside the blacksmith's shop.
USPS must provide universal service. UPS and FEDEX only provide service where it’s economical to do so.
In those days all you had was Railway Express. They didn't do delivery so if you expected a parcel you had to go to the train station to get it.
BTW, railway express was so persuasive with Congress that they managed to keep the Post Office Department out of the business of larger parcels even after passenger rail service had been pretty much discontinued ~ (the baggage and express cars on passenger trains carried the Railway Express parcels).
That is what incentivized the Post Office Department's employees at Headquarters to quit and buy UPS which they reconfigured into the sort of parcel and express package service they'd developed for the Post Office Departent.
So, "why" ~ that's "why"!