Why are rear wheels dished?
The Bicycle Wheel by Jobst Brandt touches on this:


Some rear hubs that have done a version of this include the Ritchey Zero or Z-hubs. The road Zero hub had a 21.3mm center-to-right flange measurement and 28.6 center-to-left, so combined with a 3.65mm or so offset rim it was dishless. (It didn't need an asymmetrical frame, but moving in the left dropout is secondary to whether this sort of idea can work.) These wheels do function and didn't seem to have any grievous reliability problems, so maybe they should have been copied more, but they also don't push the concept all the way to total symmetry.

I believe the deal with this is that there are some physical reasons why if you take the center-to-right flange measurement off a road hub and say, "Okay, if that's strong enough for lateral loads in one direction, then let's make the left one the same," you wind up with a wheel that buckles easily from lateral loads in either direction. In other words it's not just a matter of what bracing angle is good enough per side; in the highly tension-disparate wheels we have, the better bracing angle side is somehow making the whole thing more stable.

I haven't done the deflection tests that would be needed to test whether this is true, but what I have done is build a number of SP generator hubs with their dainty 23.5mm per side center-to-flange dimensions. Those wheels have the unlovely trait that when you do routine side-loading on them to reduce spoke windup, you can feel that they're extremely laterally flexible, and it's easy to induce a "column buckling" type failure if you're not careful, something I have no trouble with in other wheels. People ride them and they work, but from what I've experienced it's clear to me that once you're down to bracing angle numbers like that on each side, the wheel threatens to become too unstable.

Another piece of the puzzle is that, as with so many aspects of bike component design, competitive and sport use is the economic backbone of the industry and so what works there drives what gets made. How it applies here is that existent rear road wheels have been strong enough for competitive use under athletic riders, so doing anything about their lack of strength from being so heavily dished has never been seen as a priority by the mainstream, or worth compromising anything else to fix. Virtually no advantage is ascribed to increasing the strength of the wheel against vertical loads, so why sacrifice any lateral strength to get it, even if it were just in one direction? Were this not the case, offset rims would be ubiquitous, because they work against the problem in a manner that gives up nothing other than some additional costs (and have become common on prefab sets).