Because the USPS wanted to set US retail postal rates without negotiating with China Post and 150 others. Other countries' postal systems wanted the same autonomy.
So the postal union made a deal: The USPS decides how much to charge for postage to China, carries the mail to the Chinese border, China Post carries it from there, and the two split the USPS' retail price according to a set formula. Ditto for every other country combination. Simple. Saves a lot of negotiating.
But it means that if, say, the German postal system is able to send mail to the American border much more cheaply than the USPS to the German border, then the USPS will subsidise the Germans. Which is the case: I sent a heavy book from Munich to Seattle this summer for €17. Then the recipient sent it back, which cost a little over $50.
The difference is starker when you compare China and the US, but comparing Germany and the US is fair. Both are high-cost countries. I think the Americans should stop complaining until they have the prices down to German levels (or to an average of rich countries if you will).