They could just as easily make a rule that bulk mail costs twice as much as normal mail, and still require senders to presort them.
Sure, some senders would then try to send their bulk mail as normal mail. Then there could be another rule that defines what "bulk mail" is, and that anything falling under that definition must be sent bulk, with the consequence being fines and refusing to deliver that mail.
Yes, this is more complicated, and I'm sure some bad actors would still be able to get through, but I expect this would greatly reduce spam.
I imagine part of the issue with something like this is that the USPS is chronically underfunded, and policing and enforcing it would be to much extra work.
Because as a condition of the discount, it is presorted by ZIP code, eliminating a significant part of the labor cost of processing it.