Because startup costs for silicon mining and IC production are enormous, while PSUs and computer cases are available off the shelf almost in every country in the world. It makes sense to find ones place in the margins vs. cost field. Avalon are in the high margin and high cost area, since they gambled on making an IC for Bitcoin mining. An integrator/re-seller are in a medium margin and medium cost area, since they won't have to design and get ICs made from scratch, but they have to assemble modules. Adding PSUs and cases adds to their cost more than their margins would justify, since pretty much everyone who would buy a unit from them is capable of buying and installing a PSU and case themselves. (Yes, yes, I know the mining rigs pictures thread provides a counterpoint, but the point remains that selling modules to miners is more cost effective for both the miners themselves and the integrator.)

The above is of course only on-topic in this thread if KNCminer really are buying their ASIC wafers from Avalon.