Ok, I once thought your premise was the answer, but it comes down to your second sentence, >" Congress wants them to be revenue neutral (not costing taxes) and "private" (they're not really privatized) but then puts all sorts of prohibitions on them and doesn't allow them to behave in a way to make them successful..."<.
No one, regardless of political persuasion, or economic status wants to pay taxes. Taxes are nothing more than a political football, in almost every election campaign from President to Dog Catcher. We all believe we pay a disproportionate level of taxes compared to________________. Yet we all have a responsibility to fund the government and programs that benefit the "greater good".
Congress, consisting of politicians recognizes both that they need to impose taxes to fund the government, and that if they advocate taxation, they put their re-election at risk...So.....you have pretend revenue neutral modifications, and pretend privatization, to please all factions...
The blame does not end with congress, there is much more to it. People frequently comment here about how poor the Postal Service, UPS, Fed Ex are. One is Federal and Unionized, the second shareholder owned and unionized, the third, Shareholder owned, most likely not unionized. Wall Street is not happy with either private carrier's performance, because you can cut only so much. So if they could get the government to eliminate the USPS, then volume at the shareholder carriers would inevitably increase, as with rates...
Think about it....follow the money especially the money for lobby activities.