The OP's got a point, and there are systems out there that fit the bill. Look at the Pentax 50-135 f2.8. It is much smaller and lighter than the FF equivalent 70-200 f2.8. The FF producers don't seem inclined to produce this kind of kit, probably for the reasons others have mentioned.
Thanks Sporting. That's what I was thinking of, it's half the weight for an equivalent image, I guess I really mean field of view. The quality may not be big white equivalent but nor is the price.
This example makes me think it would be possible to have much lighter systems that were close in quality.
I still don't think it will happen but I would like to see this more than a high megapixel camera.