Because we allow judges to hand down sentences in order to maintain the decorum of the courtroom.

> It shows in the United States there are people that can arbitrarily add months to your jail sentence for insulting them, or simply send you to jail if you happen to do it while being in the same room they're in.

Not specific people, specific officers of the court who are performing the duties which pertain to being such. You could insult a judge outside of a courtroom all you want and they wouldn't be able to do any more about it than any other private person. Even less, actually, given that the judge is a "public official" under the law and, therefore, the Sullivan "actual malice" standard applies, which makes it substantially more difficult for someone to prove defamation.

https://en.wikipedia.org/wiki/Actual_malice