Libel is much more specific than just insults. Libel entails knowingly making false statements to harm someone's reputation. It's also much harder to prove than most people think.

Someone tried to sue for calling someone a Nazi but lost [1]. In this case the defendant claimed to have been presenting a hypothetical. I have not been able to find a successful defamation case due to someone being called a Nazi in the US. Not to mention it wouldn't affect calling a black person a n____r because such a statement is not factually incorrect (as the slur is a derogatory term for a black person).

1. https://www.google.com/amp/s/www.thewrap.com/james-woods-bea...