There's no routing and sorting they have to do. The carrier just picks up a stack of letters at the start of the shift, and drops one in every mailbox.
The labor cost that the USPS has to put in for a mass mailing is a fraction of what it has to do for normal mailing.
It's market segmentation, full price service for those willing to pay it, and a cheaper option for business without enough margins to justify that.
This means that mass-mailers are being subsidized by the USPS’ implementation of this goal of nationwide first-class mail, because presumably they are paying less than the actual cost of mailing their advertisements. I suppose we could go a step further, and say that high-density customers are in fact subsidizing mass-mailers, which I think most people would agree is not a reasonable subsidy.