Why do some cisgender people find the term “cisgender” insulting or offensive?

I don’t really get it. It’s just a term referring to those who aren’t transgender. Sure, the word may sound strange and overspecific to them and some people may use it as an actual insult like “cis scum,” but there’s no point of making it a slur. The word is at the same level of the word “transgender.” Words don’t become slurs just because a few people use them as such.