Most if not all their buildings are federal assets. I know, lame reason but I don't want to give them an excuse for yet another costly neverending bureaucratic plan before they figure/agree on whom to resell to, still with cronies dwelling around. So, that would save time and money to just leave it that way, in my guess.
Plus, history shows that with a pay-as-you-use fee system for mail stamps, the public postal service in most countries (*) has never been the biggest sucker of wealth. It's everything else around it that brings things down, in my understanding.
But heck, I suppose you're right - even THAT could be privatized, in the principle.
* (and I don't see why the U.S. would be different)
"Cyril" pronounced "see real". I code stuff.
"To study and not think is a waste. To think and not study is dangerous." -- Confucius