Being insulted is subjective. There is not any right to not be insulted thus insulting people is a great example of the sort of speech that free speech rights are intended to protect. I can say what I want, even if I intend it to insult someone, think about it, if no one was insulted or offended who would try to silence that speech?
If someone chooses to be jerk and debates with insults then they may not convince whoever they are arguing with but if we make that a crime then anyone can silence anyone else by claiming to feel insulted.

I made a meme that explains it: