I think I understand what hearty is trying to say so let me see if I can explain it further.
First of all let's just explain what f/numbers mean as they relate to aperture size. F/numbers themselves (the 2 in f/2, the 5.6 in f/5.6...) are not the actual size of the aperture opening in a lens. What f/numbers do is they provide a ratio to determine the size an aperture opening based on varying focal lengths. This provides a universal way of using the same numbers across different lenses and still mean the same thing.
Example, a 24mm lens set at f/2 has an aperture opening of 24/2=12mm and if you set it to f/4 then the aperture becomes 24/4=6mm. While if you set a 50mm lens to f/2, its aperture size becomes 50/2=25mm and if you set it to f/4, its aperture size becomes 50/4=12.5mm. Now both the 12mm in the 24mm lens and the 25mm in the 50mm lens are equivalent apertures as far as the amount of light that passes through. Same goes for the 6mm in the 24mm lens and the 12.5mm in the 50mm lens. But instead of using different aperture sizes for different lenses now we can use these ratios or f/numbers to simplify operation across multiple lenses. This is basic aperture knowledge but I felt I had to point it out to make my next point.
Next let's take a quick look at how apertures are designed in zoom lenses. In any zoom lens (variable or constant aperture) the size of the aperture at the tele end is always bigger than it is at the wider end. However, in constant aperture zooms, the difference between the aperture size is bigger compared to variable aperture zooms. This implies two things. First, that the limiting factor in designing zoom lenses is the tele end and not the wide end. Second, that constant aperture zooms can theoretically accomodate bigger apertures at the wide end.
Example, take the 24-85mm f/3.5-4.5. At the tele end, its maximum aperture is 85/4.5≈19mm. At its wide end, the maximum aperture becomes 24/3.5≈7mm. The difference between the size of maximum apertures at tele end and wide end is 19-7=12mm. Now take the 24-70mm f/2.8. At the tele end, the maximum aperture is 70/2.8=25mm. At the wide end, we get 24/2.8≈9mm. The difference between the sizes of maximum apertures here is 25-9=16mm.
So if the limiting factor in the 24-70 f/2.8, for example, is the 25mm aperture opening at the tele end, then what's to prevent manufacturers from designing a 24-70 f/2-2.8 or even 24-70 f/1.4-2.8? Because even 24mm @ f/1.4 has a smaller size aperture than 70mm @ f/2.8 (24/1.4≈17mm < 70/2.8=25mm).
Hearty, I'm not a lens design expert and don't know much about lenses beside a basic rudimentary knowledge but if I had to take a guess as to why no one makes such lenses, I'd say it's because the wide end would suffer considerably if they did. I think the problem here is that we're assuming that the aperture size at the tele end is the only limitation and ignoring any possible undesirable effects at the wide end. Current constant aperture lenses almost always will have poorer performance with aperture wide open at the wide end than at the tele end and specially at the corners. Making the aperture bigger at the wide end while keeping the size of the lens the same while at the same time maintaing the same quality is probably very hard from a lens design perspective.
So in the end is it possible? Probably so but probably not as feasible as we'd like it to be. Canon's new 24-70 f/2.8 II with its stellar corner performance would probably be the best candidate on the market today for something like that. However, it already costs so much more than its predecessor and it now has an 82mm size filter instead of 77mm. Besides, if they did make it into something like a 24-70 f/2-2.8, I would think it wouldn't be able to give us the great performance at the corners that it currently has.
