junk mail
And therein lies another part of the problem: USPS accepts boatloads of that crap and even gives it preferential pricing, to where it's inclusion becomes a justification for more deliveries whereas the revenue generated doesn't cover costs. Price it at - or closer to - first class rates, and see how that works. It's a self reinforcing/defeating matter.
I haven't looked into it, but I would assume junk mail and political crap makes up the bulk of the mail and therefore their budget.
Price it like any other mail. No special treatment. No one rides for free.