The Post Office leaves themselves open to this, because their site is at a level of tech that makes it hard to distinguish from a spam site from 5 years ago. It's too easy to make the phishing site look more legit than the actual site.
j/k, of course
The problem is that USPS is an easy mass attack target since nearly 100% of people get mail.
If you want to phish people, you need to cast a wide net, and this one is the widest possible one.