Why do people try to argue with traffic wardens?

It's pathetic to behold

They’re embarrassed about doing something really dumb so are determined to try and claim it wasn’t their fault.

You just defined Stultus stultus brexitus, sails. 

Quite often if you plead your case in a civilised manner, and they haven’t finished booking you, they’ll give you a break.

Because they are the personification of "The Man" and not hiding behind a phone system. It's easy to yell at someone in front of you. Most people are stressed and angry these days.

They will not ever give you a break. They might give you an extra ticket for being that person who shouts EXCUSE ME! EXCUSE ME! 

Also the same with forgetting to swipe in with Oyster at City Airport or wherever.  If you just say sorry I forgot to swipe in, they generally just let you off.  The people that get all worked up tend to get a ticket.

I’ve talked my way out of parking tickets on two or three occasions that I can recall, always before they’d finished entering the ticket into the system.

I once lost my card and the man at marble arch just let me out and I still remember this as a standout kindness moment

No way, laz, you're what they call a three ticketer. Excuse Me guy who also thinks he's One Of Them.

Because they have no concept of the consequences of their actions, and have never been said "no" to and DON'T YOU JUDGE ME, YEAH, I KNOW MY RIGHTS, FAM, I CAN PARK WHERE I WANT, INNIT?