It's nothing to do with it being more taxing. It's to do with the fact that video editing is a real-time task. If you put your Mac to one side and work somewhere else then you are not doing your job as a video editor, you are doing another job. Our guys generally have Final Cut Pro open all day and work on anything between 2 and 10 video streams simultaneously as well as audio tracks. Are you thinking of 3D rendering? It's very common to leave a final render going and do something else.