why people get offended?

[Audacter]

Because they shouldn't be used ever? And most people see them as derogatory.

^
If you say it to a friend it's different. Also it's risky doing over the internet as body language and tone of voice make a ton of difference when people receive those comments.

[Fleet Admiral Zapington]

Because we live in 2015 where everybody is PC as fuck even if it has nothing to do with them. Honestly if you get offended by words that easily, you should delete Internet Explore and throw out your computer. Do you guys also get offended when you hear two guys calling each other "N1gga" in real life? You don't.