Because job itself not enjoyable. Because every job, from CEO to VP to Director to Manager to Trash Pickeruper to Fishmonger (I've done all of these myself), has tons of stuff that is not at all fun. It's not at all fun to do customer support every night from 1am to 8am -- and then go to work.
It's not at all fun to refactor code or fix someone's crappy bugs or rewrite some piece of code no one will ever see. It's not fun at all to get on a plane to Pittsburgh or Joplin or wherever, stay in some crummy hotel, hurt your back in the crummy seat on the crummy flight, to go hear from some customer complaining about why they are angry and hate some part of your product.