I would guess that is because it doesn't always mean much. For instance... many of our regular users never turn their computers off... and in fact will "be on" as much as 18 hours between any real activity.
If you think about what is really happening with web sites, users are not 'going' to your site, they are downloading pages of content and data. Lots can happen in the office or home between page request. They can go to lunch, get coffee... take a nap. None of that speaks to any sort of worhthwhile engagement metric.