Because to a user that barely understands tabs as it is, it's simply not going to fly. And yes, many nontechnical users do struggle with the idea of tabs.
It's not going to happen because that would involve cutting a good chunk away from the side of the content area, standing in the way of pages' Manifest Destiny of extending from the left side of the screen to the right.
Having pages like that is a shitty way to browse; people who do it are doing themselves a disservice by reading lines at that width, and screens are wider than ever, but people love full screen windows.
