Because they are government employees, and they feel safe and entitled.
It’s just like the DMV. It because such a popular culture joke and trope that DMV workers are slow, rude, and unhelpful that a lot of states reformed their motor vehicle divisions.
Don’t get me wrong. There are probably a LOT of postal workers who are very nice people. I’ve known a few.
But when you have zero fear for your job, your mentality changes over time. There’s a lot of “why should I go the extra mile” and “that’s not my job”
I’m retired from the post office. I was a clerk not a carrier. I saw many employees as just the same as private companies. Some were very hard working. Others were very lazy and did little. At my installation it was open 24 hours a day, 365 days a year. Three separate shifts. I had my own days of depression during the thirty years I worked there. The post office processes millions of pieces of mail every day. I’ll be very curious to see what changes Trump makes.