I know it's because of the space taken up by the rear cassette. But why not just make the hub narrower, and not make it asymmetrical? Sure, the drive-side hub flange must be moved inboard to make room for a big cassette. But why not move the other flange inboard as well?
The spokes must be at an angle to provide lateral stiffness to the wheel. However, the wheel must be in static equilibrium, so on a dished wheel with different spoke angles on each side, the drive side and non drive side spokes must be in different tension, or else have different diameter or spoke count. This is well known. But why go to the trouble? Why not move the non drive side flange in as well, and use the same spoke tension on both sides?
Suppose on the drive side, the spoke flange is offset 20mm from the hub centerline, but on the left, the flange is offset 40mm. Obviously, 20mm of offset is sufficient offset for the drive side, in order to impart sufficient strength and stiffness to the wheel. In fact, I postulate that the larger offset of the non drive side is unable to add additional stiffness vs. the 20mm offset of the drive side because the wheel must be in equilibrium. Any additional offset of the non drive side must be compensated for by looser spoke tension, anyway, giving no benefit. So why not just make it 20mm on both sides, and reap the benefits of a shorter, stiffer axle?
It cannot be true that dish provides a laterally or vertically stiffer wheel. The wheel must be in equilibrium, so the stiffness of a dished wheel should be the same as a wheel where both flanges were simply the same distance from centerline as the drive side. In other words, the drive side is the limiting factor. The only advantage I can see to dishing is that the lower spoke tension on the non drive side might allow thinner (but longer!) spokes and thinner hub flange, saving weight. But this seems unlikely to compensate for the weight savings that could be had with a shorter axle.
Another possible reason is an aesthetic desire for a laterally symmetrical frame dropout spacing. But in a world of Lefty forks, is this really a barrier?
I thought of this after seeing a heavy duty touring bike with an un-dished rear wheel.