Because polite society deens them so.
Why are bad words bad?
No "Bad Words" was a great movie and not bad at all!
Words themselves aren’t bad, words are just specific sequences of letters or verbal sounds. It’s the meanings we attach to certain words that are bad.
Numbers aren’t bad either, they are just symbols for particular values, but ask people about the number 13, or 666. Bad numbers, say some. It’s the intent we apply to the tools of our language that make them good or bad.