I think everyone is severely underestimating the size, weight and price of such a lens.
The closest lens to compare to is the Sigma 200-500mm f/2.8 (three feet long, one foot wide, 15kg, $25,000), which has a bit more on the long end but is still only less than 1/5th of the zoom range of a hypothetical 28-400mm. It's also priced in Sigma dollars from the mid-90's so compare that to current Canon prices and the price would be a lot higher for the hypothetical lens (ie hundreds of thousands).
Plus there's a few other issues too - a lens that big needs a lot of gears and motors to drive the zoom mechanism which makes AF a lot slower. My guess is that, like the Sigma, it would also need it's own independent battery as well.
And even if you had this lens, how are you going to shoot with it? You'll probably need a geared industrial head mounted on some very sturdy sticks (so add another hundred grand to the equation).
All things taken into account, it's easy to see why they do not make such a lens. A 30kg + lens that costs more than your house and requires a three-man team to set up and operate? No thanks!