You're forgetting the power brick. Pico PSUs just do the DC-DC conversion; most of the size of a PSU is determined by the components that do the AC-DC conversion, which is what the power brick does. In ATX/SFX PSUs, it's all consolidated into a single unit.I'm not so sure it's a technical problem, but more of a cost/demand. I'm in no way educated to say it's actually possible but looking at a picopsu which gives 120w. Surely you could fit 10x those components in the volume of an SFX case?