Because their current competition, the USPS does so as a service that's paid for by your tax dollars.
Once that goes away, or is sold off, none of them have the incentive to continue to do so.
And in the end they will all eventually collude on raising the process for all of their services, just like every other 'not a monopoly' corporate collective.
It was never about saving money. It was always about dismantling the government so that the oligarchs could carve out their own personal fiefdoms out of whatever's left.
Thankfully, most of that shit is still on magtape, so actually have to work for it.