Why no zoom lenses lower than f2.8?

remember this is actually f/2.8, a ratio of focal length to aperture. the current 70-200 f/2.8 is under 9cm diameter. an f/2.0 would require about 10 cm diameter, an f/1.4 zoom to 200 would approach diameter of the 600 f/4. massive weight and prohibitive cost would be major factors.