The general notion is that at wider focal lengths, IS is not necessary as the effects of vibration of the lens itself are barely registered unless the movement is significant (i.e. a significant bump, vs. what is caused by hand-holding or manual focusing). This is because the amount of information captured by a lens at a wide focal length is quite tremendous, and magnification is lower. The magnification factor affects vibration as much as it does detail.
Given that, I do find it a bit odd that the 24-70mm lenses do not have some kind of image stabilization. While 24mm is pretty wide, 70mm is close to the point (100mm) where IS is certainly useful. I think it is only stranger that lenses in the 24-105mm range do seem to regularly include IS. In contrast, I am not surprised at all that ultra-wide zooms like the Nikon 14-24mm lens or the Canon 16-35mm/17-40mm lenses do not contain IS, as their widest focal lengths are still well within the range where IS would provide minimal improvement at best, and otherwise be rather useless and pointlessly costly.
It may simply be that the benefit of IS at 70mm is just not enough to justify the additional complexity and cost. It is also possible that adding IS to such a lens would also reduce optical quality, again for minimal gain. I general, I think that brand-name 24-70mm lenses are some of the best quality zoom lenses money can buy.