You can change it, as @tophee says (/admin/customize/text content – you can then search for the word you want to change or the name of the thing that contains it; it’s easier than you might guess).
I think that people don’t change it because those are words that are fairly universally understood. I worked with a client just yesterday, though, who was interested in changing “category” to “Chapter” (of their large organization). I couldn’t explain quite as cogently as @tophee did about why it’s a bad idea.