Because it fits their character. I do try to take my use of language seriously and I am very conscious that authors have complete control over what their characters say and so I very much try to avoid using character as an excuse, but at the same time it feels genuinely inauthentic to me for every character to follow the exact same list of social taboos. My characters come from a range of backgrounds and cultures, and which words and phrases they avoid or don’t avoid are part of the way they express that.