The quick and easy answer is "pointless and too expensive". Matt's answer also covers a part of it.
Pro-bodies are, as Matt points out, mainly bought by people who have spent some time with photography and hence already have lenses. They will not necessarily want a new lens.
Wes also correctly points out that the 5D MK III is sold with a lens as a kit - as was the 5D MK II, with the 24-70 or the 24-205 lens.
However there is another reason and that is complexity:
The more combinations you offer, the more difficult it becomes to chose - and the more choice, the less people actually buy. (Source: http://www.ted.com/talks/sheena_iyengar_on_the_art_of_choosing.html , http://www.ted.com/talks/sheena_iyengar_choosing_what_to_choose.html )
So in this respect having too many options is not good for selling cameras.
In fact, if you bought a 5D MK II with a 24-70, chance is that you got two boxes - two individually boxed products that are offered at a lower price together, an "artificial kit". Then again, the very moment you buy several articles, you are in a good position to ask for a discount (at least in Germany) and are quite likely to get a bit off (the more you buy, the more likely).
Hence the advantage of a kit is reduced.