That's rather like saying you have no reason to expect privacy because you rent an apartment instead of owning a house
No its not, not even a little. The home owner/apartment dweller is paying for services. Interestingly enough, landlords do have access to apartments, given notice.
The employee is getting paid for services, and most certainly part of the employement contract makes it clear that work equipment is works property to do with whatever it wants.
The post office is a special network, with special laws governing it. Its not a company its a government organization. FedEx and UPS are not bound by those rules except by volunteering to do so. Its a federal crime to screw with someones mail, its not to do it with fedex or ups.
Rental agreements are another special case as you are basically 'the owner' as long as you honor your contractual obligations. I assure you, the instant you don't pay your rent and get evicted, you've lost every bit of 'privacy' you had, and they'll be cops at your door to tell you just how much privacy you've lost as they physically remove you from the property so someone else can come in and go through your shit, throw away stuff, sell stuff, whatever.
You really are confused about the way the world actually works.