Short answer: The digital photography revolution has pretty much eliminated any idea of a standard display size and viewing distance. Depth of Field calculations are always based on several variables including the display size and viewing distance.

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 eyes, 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.

The DoF scales on most lenses assume output to be from a 36mm x 24mm film/sensor onto an 8x10 print viewed at about 10-12 inches by a person with 20/20 vision. If you display larger it decreases the DoF of the exact same picture file. Different sensor sizes (and thus different magnification ratios to wind up with the same viewing size) also change the circle of confusion. With modern pixel peeping being what it is, assuming the old standard CoC upon which the lens scales are based is highly optimistic. Viewing a 20MP image at 100% on my 23" HD (1920x1080) monitor is the equivalent of printing at about 60 inches by 40 inches!

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. The same goes for lenses that may be used with different cameras that have different sensor sizes. The DoF scale for the same lens will be different for an APS-C camera than it would be for a Full Frame camera. Assuming the standard circle of confusion used for an image produced with a 36x24mm sensor, displayed at 8x10, and viewed at 10 inches by a person with 20/20 vision will accurately predict perceived DoF for most images is too broad in today's market. 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). Note that all results from DoF Master assume an 8x10 size image viewed at a distance of 10 inches by a person with 20/20 vision. If you want to account for differing display sizes and distances, you can use the Flexible Depth of Field Calculator from Cambridge in Colour and click show advanced to enter those variables.