Because they are not interested in your answer. They are interested in you listening to their self righteous claims.
Before they say the second word, have your back them while walking away. Really, just turn and walk away while they spew their bullsh!t, and have fun with it. It'll turn your frown upside down in a second. Sometimes people need to be slapped like that to drive the point home.
That works too.
Not always, but occasionally!