In a zoom like a 70-200/2.8 the whole of the front element isn't used at 70mm (or other intermediate focal lengths). The effective aperture is controlled by the internal zooming optics.
The reason is probably to maintain image quality and/or to maintain a constant aperture, which is something photographers used to want since before the days of auto exposure it meant you didn't have to change exposure settings when you zoomed.
Of course there are many lenses which change aperture, such as the 70-300/4-5.6. In theory I suppose you could make it a 70-300/1.4-5.6 since it has something like a 55mm front element. However when you consider the cost and complexity of a lens like the 85/1.2L prime, you can get some idea of the difficulty of making such a fast lens. The image quality of the 70-300/1.4-5.6 would likely be awful at 70mm f1.4 (or even 2.
, especially if the cost was low.
For 35mm lenses, you simply don't see zooms faster then f2.8 because of the complexity (and cost) of the optics that would be needed to make an f2 zoom with high image quality. If you drop the frame size to even smaller than APS-C, such as the Olympus "4/3" format (18 x 13.5 mm), then you can make an f2 zoom with decent optics like the Olympus 35-100/2
, though it will cost you $2500! With even smaller formats such as are used on camcorders, even faster zooms are possible.