I think that it is because they have to deal with scum all day long. Traffic officers get a lot of grief from perps who are angry with themselves for getting caught and take it out on the officer. And other policemen see the dregs of society as they are called to fix the messes that evil or careless people make.
I got a speeding ticket on my motorcycle (1978 Yoshimura Honda 400-4), and I was having a respectful dialog with the officer as he wrote the ticket. He tore it off, gave it to me, and waited for "it." But I said, "Thank you, Officer. Have a nice day." The man nearly fainted.
When there is a dispute between a LEO and anyone else, I tend to come down on the side of the officer. If an officer is wrong, it's news. When he's right, it's so commonplace that no one notices.
Thank God for these people who protect and serve us.
My honorary uncle, once a cop on Glendale PD in SoCal, told me that cops see three kinds of people. Cops, civilians, and scum. After they’ve been on the street a bit too long, they only see two kinds of people. Cops and scum. This was over thirty years ago, so I don’t see the situation as having improved in that time...