FedEx does not have a monopoly on non-government delivery services. You should be comparing USPS volume with the total of all the major players, including companies like Amazon who run their own courier services.

"No company has the volume the USPS does either, " Might be worth mentioning that USPS has a government-granted monopoly on mail... so it makes sense it'd have a higher volume. If UPS or Fedex were allowed to deliver mail, then the stats might be very different.

This doesn't really address the political concerns OP says they're most interested in.

FWIW, in the UK the Royal Mail's 350-year-long monopoly on postal services was abolished in 2006. Currently only about 50% of mail is processed solely by RM from collection to delivery. Most of the remainder (sent by businesses, not private individuals) uses private companies for the "long haul" part of the mail distribution, and those companies subcontract the "last mile" of delivery to individual letter boxes back to Royal Mail.

@lly - the last paragraph does; it's a facet of freedom. One of the easiest ways to subjugate humans is to keep them in the dark. If the OP had learned about the Homestead Act the same day, in third grade, that they learned about the PSA, and later, that an annual net loss of $9B was slightly more than one percent of our military budget, then they'd understand why no one cares, and those that do will scream at you from roof tops until you regret asking.

@Mazura No, it doesn't. "Desubjugating humans", even if that were a serious issue instead of rooftop howling, isn't a political consideration. You may mean something like "it helps Democrats in elections, so they oppose mucking it up" but need to state it in those terms.

@ NPSF3000, as was pointed out above, that would be a massive expansion of their services. As @Rayathal pointed out, aside from the USPS handling as many packages as FedEx and UPS combined, they also deliver ~150-200 billion letters per year. In addition, FedEx, at least, uses the USPS to handle uneconomical deliveries. For example, they 'deliver to Maine' in the sense of dropping off packages at the main USPS depot in Maine, and having them do it.

@NPSF3000 Is it illegal for me to write a letter and pay FedEx to deliver it?

On the topic of capacity - if the USPS were to be privatized, the company that acquires it would acquire the infrastructure for it too, so I don't see it as a factor. And furthermore, I don't see exactly how those factors play into political disinterest for privatization

@user253751 No, it is not illegal to use FedEx to deliver a letter. You will however need to put that letter in packaging that FedEx will accept. The packaging that FedEx will accept exceed the maximum size that the USPS considers to be a "letter." Your 8.5x11 inch sheet of paper that easily fits unfolded in a 24x34 cm FedEx cardboard envelope is not a "letter" per USPS rules. The largest packaging that the USPS deems to be a "letter" is 6 1/8 inches by 11 1/2 inches (15.6x29.2 cm). The oversized packaging used by FedEx et al. is how they skirt the USPS monopoly on letters.

@user253751 in addition to what David Hammen said it is 100% illegal for anyone but USPS to deliver to a mailbox, but if leaving the "letter" on the front porch or something is possible.

@DavidHammen Is it illegal for FedEx to accept letter-sized packages?

@BarryDeCicco " as was pointed out above, that would be a massive expansion of their services" And? What point are you trying to make? "they also deliver ~150-200 billion letters per year." Which is a nice revenue stream.

@ NPSF3000, what I meant was that (a) neither FedEx nor UPS would be able to handle USPS' package services without an extremely large (and expensive) expansion. Basically they'd have to recreate the USPS. The letter business would add ~150 billion items to deliver each year, when those two businesses are in the 3 and 20 billion/year zone. That's a massive volume and complexity which those two companies are currently unequipped to handle. Note that FedEx doesn't really deliver to Maine, but to one address in Maine. They don't want to be the USPS.

@BarryDeCicco even if take that argument as true... it seems completely unrelated to the question at hand. It simple argues claiming that USPS is big, and therefore a replacement would also likely be big. That speaks nothing to the point about whether the USPS or a replacement must be public. Or in other words, arguing that change couldn't happen because that would be change - simply isn't a logical argument.