Developing power supplies for Bitcoin miners is in a class of problem called "bitch on wheels". This however is not an unknown issue, it has buried a number of other companies over the years and anyone who has designed high power supplies knows the joy of watching smoke and plasma balls rise from 300 amp IGBT circuits.
I think however that people designing boards tend to be simply willfully ignorant of the issues of providing 500 amps of power at half a volt and thus they screw up the same thing over and over again. At least on a SHA256 chip each engine is pulling constant power through the hashing cycle.
You're right. Powering this type of ASIC is a real challenge. However, there are solutions available for CPUs and GPUs - typically, these are complex multi-phase systems with sophisticated feedback systems carefully tuned for the precise application. While you can buy the voltage controller ASIC off-the-shelf, you still need the inductors, switches, and compensation network, as well as a suitable circuit design.
One of the things that struck me when I first saw the PCB layouts for the alpha board was that they appeared to be using off-the-shelf , but very expensive ($50 each in 1k quantities), DC-DC modules for each group of 4 ASICs. This suggested to me that they either didn't want to fund the design of a DC-DC converter based around a CPU/GPU VRM ASIC, or that they did not have the expertise to be able to build such a converter.
The latest spins of the board appear to use Altera monolithic DC-DC converters - an impressive ASIC with integrated switches and integrated inductor! Again, suggestive of the fact that there is no will or capability to build a DC-DC solution in house. The Altera converters are even more expensive - $19 each in 1k quanitites - with 1 converter needed per viper ASIC. The other issue is that they seem desperate to keep the cost down, they've said that the viper ASIC needs 15A @ 1.2 V - so it's rather optimistic to use a monolithic DC-DC converter rated at a maximum current rating of 15 A.