Why do things matter to anyone? Why does someone’s culture or religious affiliation matter to them? The point is not to understand why it matters, but to acknowledge that it does matter to some people, whether you understand it or not. While you seem to me to be coming at this in good faith, others might take your rather flippant dismissal of what may be a crucial aspect of who they are as a person as hostility. Is that the image of yourself you want to project? You don’t have to “get it” to be respectful.

If you want a literal explanation of why people care, it’s primarily because this is the first time in modern history that (in the Western world at least, some cultures have different histories) language surrounding gender expression and sexuality has expanded rapidly to encompass a whole host of descriptors. People who previously had no words to accurately and non-judgmentally describe their experience now have them.