edit: Also also, they have to go into a physical post office and be observed trying to steal your account. Given how it's quite possible to steal accounts via social engineering, this seems like an improvement in security, not a reduction.
The social engineering attack surface of my account currently consists of a handful of support contacts at my ISP, who have been trained to deal with computer security. If you allow any USPS employee to access your account, you've suddenly increased the potential attack surface by several orders of magnitude.
(2) An entity you're directly paying to may go out of business, sometimes due to circumstances beyond their control. At least state-sponsored entities don't do it so abruptly in most of the "civilized world".