The tennis court is a specific size. The net is a standard height. You can remove the net and hit the ball, but then you're not playing tennis.
Similar conventions apply to resume writing.
You can make up your own rules as you go along. They are its only reason for being. Things aren't dropped in helter-skelter. Information falls under easily understandable general headings, which makes it easy to find facts. It's broken down into sub-headings.