First, a word about what depth-of-field is and is not:
In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out
of focus to one degree or another. What we call DoF is the area
where things look, to our eys, like they are in focus. This is
based on the ability of the human eye to resolve certain minute
differences at a particular distance. If the slightly out-of-focus
blur is smaller than our eye's capability to resolve the detail then
it appears to be in focus. When you magnify a portion of an image
by making it larger or moving closer to it you allow your eye to see
details that before were too close together to be seen by your eyes as
separate pieces of the image.
Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth
of field gets narrower as the near and far points where your eyes can
resolve fine details moves closer to the focus plane.
Since depth-of-field is dependent upon viewing size and distance as well as the visual acuity of the viewer it is hard for a camera to indicate depth-of-field if it doesn't know what the display size of the photo will be. Assuming the standard 8x10 viewed at 10 inches by a person with 20/20 vision is probably a little too broad. Because of this it has never been a priority to the users and designers who advise major manufacturers on what features are desired in upcoming models. Most of the photographers in those groups are advanced enough to have a feel for the distances/focal length/apertures they use the most and probably don't see a need for it. They also understand how to use distance scales on lenses that are marked well enough to be usable. Unfortunately, lenses with usable distance and DoF markings seem to be getting more and more uncommon. This seems to indicate the market in general doesn't demand such a feature.
Although it is not in-camera, these tools from DepthOfField master.com are simple, easy to use, and the price is right (free).