Because no one is insulting the people named “Karen.” They’re giving other people (obnoxious, self-entitled, usually racist people) the title “Karen,” which is not the same thing.
Consider it this way: If someone were born and the parents happened to name him “Jerk,” should he get offended every time someone called some jerk a “jerk?” No, because they wouldn’t be referring to him. His name just happens to be a homonym of the insult.
I used “jerk” instead of a real name to highlight the difference between the name and the insult, but the same goes of course for actual names like “Dick.” Calling someone a “dick” is not insulting someone named “Dick.”
A “slur based on something someone was born with and cannot change” is specifically about insulting that person, not about calling other people by a word that is the same as someone else’s name.